The most serious misdeed
Apart from the fact that cheating is wrong, the fact that someone labels you “crazy” is a major red flag. Sydney saw her four-year boyfriend plainly cheating on her with another female, and instead of confessing, he chose to blame her for her behavior.
He was attempting to manipulate her and weaken her feelings in order to get out of the mess he had created. That’s a huge red sign if we’ve ever seen one.