If you trust your girlfriend, it shouldn't be an issue. If you don't trust your girlfriend, you have bigger issues than who she hangs out with.
[This is coming from a 100% personal experience standpoint. I'm currently in a long term relationship and am quite happy with it. That said, we have a mutual male friend - who I've known and been friends with for longer than I've known my BF - who has had a "thing" for me for years. My BF knows this, I know this, he knows this. I would never act on his feelings for me and I don't think he would either, but my boyfriend is 100% comfortable with telling him to shut up if conversation gets too, erm, risque for his tastes.
The situation could be weirder, I suppose, if all three of us weren't friends (i.e. if I knew the male friend but my BF didn't), but it still comes back to trust. I have many male friends, and my boyfriend has never expressed worry that I spend time with them - nor have I given him cause to worry.]
i think if anything its a sign of disrespect if she continues to hang out with him if youve made it clear to her that its not okay with you. if things are clearly inappropriate, or its imminent they will be, dont allow it to continue
If there's no sign of overtly inappropriate behavior - and he didn't mention any - then he really has no right to say it's "not okay" for her to hang out with a friend. It's not disrespectful on her part unless she explicitly goes for a disrespectful angle ("I don't care how you feel, I'm hanging out with him anyway!").
If the other guy is making blatant moves on his lady, then yes - he should absolutely tell her that it makes him uncomfortable. Approaching it from an "I don't want you to hang out with this dude" position, however, is honestly more likely to make her want to rebel.
Most girls will not want to feel like you're "allowing" or "not allowing" them to do something, relationship-wise.