Maybe stabilized whipped cream with very little added icing sugar...
If you add a dash of salt to your icing...it cut some of the sweetness out
Replace 1/2 of your shortening with margarine or salted butter and it is not as sweet.
Simply add less sugar, that's what I do. It will be creamy, less sweet, tho not crusting so keep that in mind.
I don't understand why customers make that request in the first place. I'm a baker and I own a business. I have recipes that I use for my business. If you (customer) don't like how they taste, you really should find a baker who uses recipes you LIKE instead of asking me to change mine! 
To the OP: You could spend a lot of money and waste a lot of time trying to come up with a "less sweet buttercream" trying to please this client, and they still might not like the results! You could certainly try an SMBC, but it's very different to work with than an American buttercream, which it sounds like that's what you normally use. If you aren't used to it, you could have trouble smoothing it and even making it.
If you still want to try to accommodate this client, and if you normally use an American buttercream....I'd suggest doing any or a combination of the following: replace some - or all - of the liquid with sour cream; use less powdered sugar and add meringue powder, which is not sweet and will help add volume and stability....maybe use 1 Tbsp, depending on how much buttercream you're making; replace some of the liquid with a little lemon juice....not enough to change the taste, but enough to help cut the sweet. It really doesn't take much at all, so start small and add if needed.
Again, even making the small changes I suggested will take you out of your comfort zone of what you KNOW works for you, so you'll have to decide if it's worth it or if you should let this client look for someone else.
I like the whipped cream buttercream in recipes. It tastes less sweet and best of all, can be left at room temp.
Do you have a recipe for this? I am also needing to do a cake that has a not so sweet icing..??
Any help would be appreciated!!
TIA
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