Zero Dark Flirty

A female "friend" of my boyfriend's is always leaving flirty comments on his Facebook page, and it's making me upset and worried. He doesn't really respond, but because he's a guy with a girlfriend, it seems that the considerate thing for him to do would be to tell her to cool it. How can I bring this up to him in a sane way?

--Disturbed

Guys also say "Hello, beautiful!" to the 200-year-old grocery store cashier, and probably not because they're angling for her to send a selfie of how she looks without her compression hose. What keeps a guy from being all "Let's blow this timeline item and go to a motel" is whether he's ethical and into the relationship he has. If that doesn't describe your boyfriend, why are you still with him? If it does, instead of saying, "Hey! People are socializing with you on a website designed for socializing!" let on that you're feeling a little worried, like by gently remarking, "That friend of yours sure is flirty" (or whatever it takes to get your worry across). Rather than trying to control him, which leads a person to rebel, you're asking for reassurance, which should lead him to put his arms around you and explain why you have nothing to worry about. This, in turn, should get the two of you back to using Facebook as it was intended -- as a place to bring people together to view videos of cats and police brutality.