Most Helpful Guy
I say relax and trust him. Unless you see some significant signs that there's something going on with them that shouldn't be, I see no problem with him having female friends and occasionally having dinner with them. My girlfriend and I both have opposite gender friends and are fine with that because we are confident that we are both committed to each other and that those people are just friends.
Most Helpful Girl
I think you should relax and trust him. Has he ever given you any reason to think he is disloyal? If not, then this shouldn't be an issue. It's just dinner to catch up with an old friend, it's not a big deal and it doesn't mean anything. Try not to overthink it.