Hopefully I don't get flamed for this, but I don't think the point is about having him come out or not but the fact that he's sleeping around with other people (whether it's men or women) behind your friend's back.
I'm sorry but for me that's the same as witnessing a crime and not reporting it, you may as well have egged it on.
How would you feel if you found out on your own that your partner was cheating on you, and then you found out a close friend of yours knew a while back and didn't tell you?
Maybe it's just me but whatever consequences comes from his actions, the onus is not on your friend or you, but on him because he brought on himself.
You do stupid things, then expect stupid things to come from it.
I suppose you'd have to check if your girlfriend already knows somehow. Some people have open relationships like that.
You said he hasn't logged on in a while so maybe he was just scoping and not actually hooking up with people. It depends and I guess without going into super investigator mode, only time will tell.
If you really think she doesn't have any clue and that in her mind the relationship is monogamous, and that you do suspect he is in fact sleeping with other people, then if it were me I'd say something out of decency and not because you, for lack of a better word, meddle for the sake of meddling.
You should certainly break the news privately and with a good amount of grace, but in this circumstance I don't think it's really just as simple as "minding your own business" because it's not like you were suspicious from the start and investigated, you came across this by chance and, if it were me, I think a person deserves to know when they are being deceived.
How bad would you feel if she didn't know about this til ages later and then is massively heart broken, when you could have potentially saved a greater heart ache. I know it sounds very 'sainthood' and I'm not saying it's your 'role or responsibility', but in a sense it kind of inadvertantly became a little bit your business by chance because you stumbled across it and not because it's your sole 'duty' to be a protector or something like that.
In saying all that, this is all speculation of course. I wouldn't do anything until I had more concrete "evidence" so to speak. Now the thing I'm not sure about myself is if you should investigate this further discretely or not. Again, I say that only because I know people would see that as awfully prying, but again if you were to try find out more I don't think you'd be doing it just because but out of genuine concern for your friend.
That's just me and again if it helps to avoid any arguments, I do sincerely apologise if I upset people, I just thought to give my 2 cents on the matter.