Should I tell my friend the girl he has been seeing has a boyfriend?

My guy friend has recently been seeing this girl. He hasn’t had a girlfriend in years and he’s a really sweet guy, but he is the type to get fixated on the first girl that gives him that much attention. He doesn’t know her very well at all, but he already seems to like her quite a bit. I found out that she has a boyfriend through another mutual friend at work, but he doesn’t know this. Is it my place to tell him, or what should I do??