Should I tell him?

So my guy friend likes this girl. But the girl has been talking to other guys the whole time they’ve been talking and has had a FWB, He’s stepped up his style and is trying to impress this girl, even planning to prompose to her. She still talks to her FWB and obsessing over her ex-boyfriends new girls- I mean calling their jobs, making fake instagrams to find & follow their pages- and things like that. When they first started talking, he was just a free meal. Now, she told him that they were exclusive but still wants to go to her FWB’s house one last time. Should I tell him? It’s starting to get out that she’s been doing all of this and I don’t want him to look dumb. Thoughts?