Opinion:
Hi, i can see that your very worried about ur gf, need to remain
calm and cool.
Have u seen the guy before?
Did he have things superior than you? (Eg.Paper qualification,
looks, character...)
By right, that guy should not do that (snatching people's
girlfriend is not right).
That guy is "Zian(4) Gu(3) Tou(2).
I've met this kind of people before, just find some chance to shame
and humiliate him, if ur gf is materialistic it's not too late to
know now.
If ur young, 2 years still ok(can still find other girl) otherwise
u may consider to maintain the relationship if u really aceept
it.