Just meet him only you can judge from meeting him what you expect.
With no sexual interest in this guy your GF is doing no wrong.It's still hard being homosexual and still frowned on by much of society.
Be a man your GF can be proud off talk to the guy, ask him about the relationship with your GF, DONT be confrontational and ask the guy about himself, and how he handles being gay.is he confidant is he happy.Talk like a guy dont be a screamy homophobic around him.
You want your GF to keep then make an effort, if he needs her more than you and your so slagging about him you will drive her away.
A mistake is always forgivable, rarely excusable and always unacceptable.
Robert Fripp