I would start out with a subtle “alright Jenn.” or “ok let’s not with this today” In a we-get-it-please-stop tone. 

Beyond that, I would tease her right back and say “so  are you actually watching these videos then? You seem to think about this a lot.” Just embarrass her right back. “Oh are you pointing out that you’ve seen Johns dick again? Haha! Hilarious.”

Step 3 would be to say “ok Jenn, this is really personal. We get it. You’ve seen his videos. We are all aware that this happens. It’s something that he wants to do and it makes him happy. Can we please drop it now? No need to hang it over his head. I don’t mind that he does this but it really is uncomfortable for people we know to bring it up.”