use blanks, (bang, racking sound, bang, racking sound) should scare off anyone..... otherwise any shotgun round that would penetrate a would be killer/raper in the winter with a coat on, is gonna destroy your walls.
I'm going to assume that by the phrase "destroy the walls" you mean to say that you're looking for a round that will not cause damage to the kids in the next room....
So find someone who reloads 20ga shells & get them to load some rounds with rock salt....
You can find some other interesting stuff too....like rubber slugs, bean bags etc. Never seen that in 20ga though, but I haven't seriously looked.
That's my idea as an answer to your question, however, I have to say...If my assumption is incorrect, then to hell with the walls! Get her some buckshot. If she has to use it, make sure she's able to use it right!! A 30cal hole in sheetrock could be fixed in a matter of seconds....getting the blood stains out of the carpet is what you should be more worried about.