Maybe whipped cream, because it doesn't have much sugar added to it, and it's got more air than buttercream.
There's also the Bettercreme (and there's a whole thread on how to doctor it in every way possible!) 
http://cakecentral.com/modules.php?name=Forums&file=viewtopic&t=601193&postdays=0&postorder=asc&&start=0
Here's the google doc for all the recipe variations of Bettercreme: http://docs.google.com/Doc?id=dgjqsjjc_4gmsq9sdh
There's also the Bettercreme (and there's a whole thread on how to doctor it in every way possible!)
http://cakecentral.com/modules.php?name=Forums&file=viewtopic&t=601193&postdays=0&postorder=asc&&start=0
Here's the google doc for all the recipe variations of Bettercreme: http://docs.google.com/Doc?id=dgjqsjjc_4gmsq9sdh
Is bettercreme low fat??
I'm not sure, you might ask Melvira who is more familiar with it than I am.
People crack me up
- I laughed when I saw this post! I know, I know, people have diet restrictions and all - but it IS icing lol! The non-dairy whipped icings are the best way to go.
I know, I know...but my friend wants to make a birthday cake for her dad who recently had heart surgery.
I did an organic "Mango Mama" "cake" (it was all organic fruit on a bed of fresh dill) for a friend of mine who was dying of breast cancer at her hubby's request (she eventually died sadly) but it was a big hit.
It was one of those "restricted diet" situations but they still wanted to celebrate her birthday. If you look in my pix at the "towering inferno" cake -- she died the day before it was due so my cake pal who was helping me on it really stepped up to the plate in helping me with it (Linda was my best friend and even though we knew it was coming, it still didn't make it any easier.) But she LOVED Mango Mama!
It can be so hard sometimes to find ways around diet restrictions - one of my best friends is allergic to ALL corn products: corn syrup, corn starch, you name it! I can't use boxes, only scratch, and most baking powder has corn in it so I have to use baking soda recipes - though there are ways around it. She is the reason I tried FMBC - powdered sugar has corn starch in it so I needed a recipe that used granulated sugar! lol
It can be so hard sometimes to find ways around diet restrictions - one of my best friends is allergic to ALL corn products: corn syrup, corn starch, you name it! I can't use boxes, only scratch, and most baking powder has corn in it so I have to use baking soda recipes - though there are ways around it. She is the reason I tried FMBC - powdered sugar has corn starch in it so I needed a recipe that used granulated sugar! lol
Wow! that must be tough for her... poor thing
I don't think Rich's or Pastry Pride are particularly healthy, they are just nondairy. I think they are essentially made of corn syrup, partially hydrogenated oils and whatnot. The healthiest frosting I can think of is a 7-minute frosting - basically just egg whites and sugar. Calories from the sugar, but no fat.
The only healthy frosting I know is the one you don't eat.
But seriously, I really don't know how you can make a traditional cake with frosting healthy. Even if you find a way to make the frosting healthy, think of how much fat and sugar is in the cake itself. You'd really just be indenial about it being healthy.
I'd say, give up on the frosting idea. Make a nice chocolate angel food cake, fill it with fruit and maybe a little cool whip lite, put a fancy stencil on top and dust it with powdered sugar (that's why you use chocolate cake, so the stencil will show up. Garnish with a few apricot roses. (flatten dried apricots with a rolling pin between waxed paper. Use these as you would use gumpaste petals to make a rose, using corn syrup as glue, secure with a toothpick until set.)
www.wheylow.com
its super expensive, but I use the sugar for all of my home baking to save me some calories 
I recently saw a Martha Stewart wedding cake design where the cake was entirely covered in meringue flowers. It was spectacular, but typically Martha complicated. But it got me thinking... true meringue is just egg white and sugar, so it has zero fat, which is great for heart patients. You could cover a cake, say a lemon cake with meringue and use a kitchen torch to brown it like a lemon meringue pie. The Martha cake was all covered with piped meringue flowers that had been baked/dried in a very low oven so they stayed white.
You might also try a vegan whipped topping... those don't have the same fat at all. Soyatoo or Rich's Rich Whip both are yummy. The Rich Whip is going to be easier to find in most areas unless you have a big health food store near by.
Most stores have Kosher areas and have Rich's Dairy free creamer in the freezer case and so i asked my store and they ordered me a case of Rich's Rich Whip. They said they didn't know if would be a regular thing, but it showed up on the shelf right next to the Rich's dairy free Kosher creamer... at $1.00 a box. Good for me!
That egg white meringue looks to be a smart thing too. If you make the kind for dry candy meringue... those are yummy. Lots of sugar in that though/ probably not good for a diabetic, unless it can be made with a sugar substitute.
That Martha Stewart meringue cake is super pretty! I bet if you smeared it out you could bake square or rectangular panels of it to cover- say a square gift box cake... 
And if she piped roses then anything you can pipe is fair game too! The sky is the limit... 
leaves swirls dots, squiggles, stripes... etc.
I'm with artscallion. Make a great cake. You can even make healthier substitutions like applesauce for some of the fat.
But I would counsel your friend to acknowledge the white elephant in the room. Frosting is inherently not good for you. Heart patients should probably avoid it.
There are spectacular cakes that stand on their own. Maybe a decorative nordicware pan? One of the ones that most of us never get because we cover them with frosting anyway.
I haven't tried these but they are in my cookbook that was handed down to me.
White Wonder Frosting
Mix in double boiler
2 egg whites
2/3 c granulated sugar
3 tsp light corn syrup
1/8 tsp cream of tarter
Place over boiling water for 6 min, stirring until it reaches 150 degrees. Remove from heat and beat with mixer until icing will hold shape.
Fold in 1 1/2 tsp vanilla.
My husband is a heart patient and yes I would give him this over a butter cream any day. Have you considered doing a traditional cake that doesn't need a heavy icing? A nice bunt with a glaze maybe? Using natural fruit juices in place of part of the sugar works wonders in the cake as well as in making a glaze.
I as thinking 7 minute frosting too. Or using a decorative pan and just dust with powdered sugar. Good luck! That's a hard thing.
Fondant is low fat too. You could make it from scratch and use brown rice syrup instead of corn syrup, rice syrup is rated much better on the Glycemic Index. But you would still have to put something under the fondant, I don't know if you can use 7 minute frosting?
Ya know a nice rich pumpkin spice cake(low in sugar like at least half of what is called for-using all brown sugar also sweet potatoes are healthy) in one of those pretty pans with just a sprinkle of powered sugar over top and a few heart healthy berries placed about. MMM now that sounds good!
Does it have to be decorated? I like the angel food with fruit idea. I also have a low fat lemon blueberry pound cake recipe that is really yummy.
With buttercreams you are looking at shortening or butter, neither would work well for a low fat diet.
There is a product I get from a German store that you mix in with whipping cream that keeps it stable for several days. It would be a lighter (less sugar) icing, but I'm not sure the fat content in the cream, I'm thinking it'd be high.
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