I think it's more of letting your desire to have your cake and eat it too (to get both girls without committing to either) to affect your judgement of the situation.its true, but you should "fix it" with Cindy, she really need some charater development.
If Ian chooses to stay for Cindy's party thing despite his own best interest, this is a big part of why Cindy and Ian get together at all -- because he is there, reliably, for her when she's at her lowest. Something he points out and she acknowledges.
If Ian isn't there, Cindy suffers her breakdown all alone, and he effectively becomes another "friend, but not really when push comes to shove". It absolutely makes sense this would dissipate any interest Cindy might potentially have in Ian.
It's also not really something you can "fix" afterwards with some platitudes that don't hold any real weight. It wouldn't be any sort of character development for Cindy to just get convinced to overlook it as if nothing has happened.