Most Helpful Girl
It's clear that you trust him but you don't trust her. That's reasonable. I'd be uncomfortable with it too. For now make it clear to your boyfriend that the idea makes you kind of uncomfortable but you respect his decision. Since you also say that you understand that he could just be doing a good deed, I'd say let him. But if anything gets out of hand, tell him you'd like it if he stopped or at least called the other girl out on her inappropriate behaviour.