The only marital indiscretion I can think of is when he gave Abraham the okay to sleep with the helper girl to conceive a child. Beyond that he’s been pretty clear on the rules about sexuality. Since your friend made the claim it falls to him to support it and point out the verse. Just ask him to cite the specific verse(s) he’s talking about so you can do some research.
If he can’t and doesn’t change his stance, or if he refuses, then he’s not actually interested in Truth, and is merely looking for a pulpit form which to spout his nonsense. If that’s the case, just ignore him on Facebook, or take the time to do some evangelizing, even if it may fall on deaf ears.
God Bless, good luck.