If he is still with his girlfriend, there is a good chance that your mutual friend would have found that out by now.
Being that she often hangs out with him and his friends, that bit of information would have most likely come out by now.
It would be difficult for every single one of his friends to stay quiet about that.
At some point, someone would have said something in reference to that, if not deliberately, then by accident.
So yes, someone would have let it slip by now.
If they all got quiet when you passed his group in the hallway, there is a good chance that his friends are all aware of his complicated emotions towards you, at least to a degree.
If he is the leader of the group, they are going to follow his lead when he gets quiet and do the same.