Two concerns,really. 1) That your bf doesn't see how absolutely bizarre her behaviour is.
2.) The fallout if he doesn't nip it in the bud soon and it's left to escalate.

This girls behaviour isn't normal in adult circles, so I doubt her reaction to being rejected is going to be a simple "oh ok then."

This needs,addressed sooner rather than later. And suitable boundaries put in place, Carefully but firmly...And your bf has to do it...she isn't going to listen to anyone else.