Someone told me that when they covered a cake in fondant, it compressed. Is this the way it always is, or did she do something wrong? What should I do to keep this from happening?

I don't think I have had that problem. But some folks here have more experience than I do so here's yer bump.

I haven't had that problem but a fondant cake is very moist.

if you roll your fondant too thick, the weight might compress a cake, but the fondant would have to be pretty heavy. My cakes are chilled and firm when I put my fondant on and I've never had that happen.