30 years ago I lived near a McDonalds that backed onto a carpark. A group of teens regularly hung around the back door of the McDonalds, leaving their fast food wrappers scattered about when they left. The store manager rigged up a speaker and started playing classical music (loudly) whenever the kids appeared. Problem solved. They found somewhere else to hang out. If your friend’s window opens, I suggest she makes use of Spotify or ABC Classical Radio and treats the street to a short concert. Perhaps she’ll have similar success.