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

That is a tough one, since you only have the information second hand. I think I would leave that one alone (maybe she is fixing to leave the other guy and you could mess it up).

Tell him if and only if she starts dating him before dumping the other guy.

I agree with @Kayak8. If you can confirm this information then I could see having a private talk with the girl telling her what you know and asking her what she wants out of your friend. Give her the chance to tie up loose ends or tell your friend herself she dates more than one guy at a time. You never know, your friend may know and be all right with just having the chance to get to know her better.

Or you can, matter-of-factly, tell him that boys and girls alike sometimes date more then one girl or boy at a time.
Maybe that will alert him and maybe he will then ask her if she is exclusively dating him or if she is dating someone else too?

Tell him – he’s your friend. At least let him know that possibility is out there before he falls hard!

You should mention it to your friend and let him decide what to do we the relationship . . . BUT, you need to be able to back up your claim and not act on hearsay.

In the situation you describe I would tell.

yeah tell him before he thinks its a serious relationship and then he will just get hurt !