So the advice is... put yourself in a position to be sexually harassed again, so you can respond to it immediately and forcefully, like a Proper Empowered Person? That's really gross.

My advice would be, next time LW is planning social time with their friend, say "Can it just be us? I'm not comfortable socializing with your boyfriend." and when asked, "He was really touchy-feely last time we hung out together. I kept moving away and he kept moving back in to stay in contact. I'm very uncomfortable with feeling like I'm being physically pursued while I'm trying to chat with you."

So yes, tell her, and frame it in terms of LW not wanting to hang out with him. Which is true. What she does after that is her call, and hopefully as a close friend she will care about LW having a good time when they're together and not insist on Groping Guy being a part of it.