All my friend's boyfriends never know that they are dating her?

They're always 'just friends' from their viewpoint. They really do think so. This current time, I asked her 'boyfriend' some random question too see if he would answer honestly. Then I asked him if he and 'Cindy' were dating. Here are his words exactly. "No-" *looks at my expression* "....I don't think so." *alarmed look on his face*. They've dated in the past and he is a really nice guy. He wouldn't do anything to hurt her so he really was being honest. This is the third guy who didn't realize he was dating someone. I kind of feel bad for them because when they flirt with other girls she screams at them and cries and they go 'What's the big deal you act like you're my girlfriend'. What can she do to make it obvious that they are dating?