I hate thin walls in apartments. She can minimize sound bleeding by putting something absorbent on the walls--either acoustic foam, blankets, decorative quilts, or tapestries.
That's an interesting idea, I wonder how many people actually do something like this for their apartment. I can't imagine a lot of acoustic foam would look good to most people. The decorative stuff sounds good, but would it work well enough to make a large difference? I kind of wish I had tried something like this though, back when I lived in an apartment. I quickly gave up playing music loudly, with the exception of listening through headphones.