Good to know your BF has compassion, just in general. She sounds like someone who has no self esteem and is desperate for human contact, even inappropriate.
It is not your or your BF job to teach her boundaries. She seems not to understand what that is anyway.
Just continue to have as little contact as possible, BF should block her though, no matter how bad it makes him feel, it is sending the wrong message not to. She may think it is an open door and will continue to go through it until it is closed. It may be the only thing she understands.
I feel sorry for her myself and I don't even know her....