It is my experience that it is best that IF your "gay CHRISTIAN friend" doesn't think it is wrong for him to be gay, THEN don't be friends with him.
Cut him off.
I think we need to clarify the difference between acquaintances and friends.
This person is different from most other sinners that don't pretend to be a practicing and faithful Christian. I would hold such an acquaintance closer to me that fit into that category than a self-professing Christian that has embraced any sin as right.
They are deceived and are in danger of a judgement worse than the infidels who have never known God.
Seriously, you gotta be careful.
Ungodly compromises to befriend someone are just as sinful as any other sin.
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Hebrews 13:23 Know ye that our brother Timothy is set at liberty