Best would be to tell your friend
Let him resort this with his girlfriend
As LZ said if she's flirting behind his back, how much would she care for him?
Who says she will be faithfull with him if they get married?
It's best to tell him about it, either they break up because she is just not worth him, or everything will turn out fine and their relationship will turn out better.
As hard as it is, it's the best thing to do
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Behold he who stands before you
Almighty (shared) Leader of the OHF
Proud member of the Ancients Club
The one True Druidic Sorcerer of the Laughing Hyena's
Last of the Ancient race known as the Red Fluffies
Summoner of Bunnies
Spam Sentinal of the Ironworks
"You can Run, You can Hide,
But you can never do both"
I'm Watching you...