Anyone in a relationship where "he won't let her have friends" (are you sure that's all friends, or just male friends, BTW?), well, that's just a big ole red flag. Isolation is often (not always) a prelude to abuse or proto-abuse.
(and I'm shouting that so that maybe you'll actually read the remainder of this post), you have an agenda - to have her for yourself. Perhaps you are seeing/hearing what you want to hear. And, even if your observations are accurate, and even if she really does need an intervention, you would be doing it for selfish reasons.
Therefore, talk to a female friend of hers. Is this friend seeing the same pattern? Is there truly something to be concerned about? If there is, then suggest to the female friend to talk to her. Not in a "hey, you guys need to break up ASAP" manner. Instead, more along the lines of "we miss you and are your friends and want to see you" manner. There are men who are not abusive jerks but who are still in the "mine mine mine" zone. He might be one of those and, if he sees he is not threatened by her friends, he'll slack off.
But he has reasons to feel threatened by you, because you have an agenda.
See how that works?
Oh, and understand something. People who we desire who are in relationships with others aren't always in a bad way. Even if that relationship seems less than ideal from the outside, understand that you are looking at it with your own attitude, your own glasses on. Things may be better than you believe. In real life, rarely are things that black and white and obvious.