Help!

Well, consider a Victoria sponge.

Or have a look at recipes at www.kingarthurflour.com as some of them are by weight and are the "lighter" style.  They do very well with buttercream, and they even tier well with proper dowels.

Use the recipes that start with the butter and sugar being creamed--I do not recommend recipes by  the method where the flour and fat and some liquid are dumped and mixed at the start.

So do a tasting with this good client with 2-3 shortlisted recipes. Bake cupcakes or small round cakes from a half recipe.  Have some buttercream and/or ganache ready but not on the cakes.  If this is a very faithful client, I would write off the ingredients and time.  You may well get MANY more orders for these lighter cakes from her.

There are several grades of "lightness" between madeira and angel food.

The American recipes have milk and baking powder which "lighten" well. The flour has to be folded in carefully in alteration with the milk to keep the crumb tender.  Be careful to not overbake these cakes. These cakes are normally baked in layer cake pans.

Then there are chiffon cakes and true whole-egg spongecakes with or without some butter/milk. These can be baked in layer cake pans as long as they are not greased.  I line bottoms with bakers parchment.

These intermediate cakes can be baked in multiple thin layers to stack without accidents to the thin slices. Again you have to watch the baking time carefully.