I have a friend getting married and would like a cake.

The requirements are:

Gluten Free

No Animal Fat

No Dairy

No Sugar

So I know there are Gluten Free cake mixes, I guess I am not too worried about that.....Its the icing.

Fondant uses sugar, butter cream uses sugar.

Any help would be great!  I may have to decline and send them to a professional.

no sugar that's a toughy -- even truvia and the stevia sugar substitutes have half sugar in them for baking -- could you use coconut sugar?

you could make 'cakes' out of honey if that's allowed (unless no animal fat also mean no animal products?  idk -- hope not)

and do fruit fillings cooked down down down so they are real firm and forget icing -- just do a fully naked cake with all the layers showing -- you've seen those?

the gluten free cake mixes are typically made with sugar

I would be tempted to say, "You realize I make CAKES, right?"

oh what was i thinking -- you could use agave (from plant) or maple syrup even for the sugar -- but it's gonna be a bit different texture which i'm sure they know but you be sure you explain if you take this -- this sounds like some experimenting is involved -- somebody's gotta pay for that too 

I'm not sure if you'll even tackle this request (I probably wouldn't... too much unpaid time involved researching) but anytime I have a client making a very specific dietary request I always encourage them to consider a smaller gluten/sugar/fat/dairy/whateverelse-free cake for the couple and then the rest of the servings to be traditional cake. They may be used to the taste and texture of this type of cake, but the the other 148 guests may not be.  And as much trouble as you will go through making all that cake, it would be a shame for everyone else to say "Eww, the cake tastes funny" and leave it uneaten on the plate.

Getting a 6 inch cake made perfectly with none of the traditional cake ingredients is much easier than trying to work with 3 tiers of the stuff.