You can't stop gossip. The best thing for him to do would be to take himself out of the area when it starts. Then people will notice and start gossiping about him. It's a vicious cycle.
As for the leaving him alone part, I'd find out what her intentions are first. Evidently, he thinks she's being manipulative so once again the path of least resistance would be to simply avoid her. But then he'll be on the run all the time when he sees her coming. If he doesn't want to be on the lamb, try EJ way and take the direct route.
Please try to dredge up some Fe for from somewhere so that you can get her to leave you be and not have the opposite reaction of her making it her life's mission to annoy you. Lie if you need to, say you have to your car in by a certain time or hey, say you don't think her boyfriend would appreciate you spending so much alone time with her. Put the onus of responsibility back on her some way.
Given your stance that you don't want conflict, I'd say the best thing for ImNotTooPopular to do would be avoid her avoid her avoid her. Put that ENTP to work and brainstorm possibilities with you. Avoiding people is a fairly common thing and not that difficult to come up with reasons for. Which is why I didn't give you any.