What was the manga? Also in that manga, was the girl aware about the truth regarding the guy (and were the readers aware that the girl was aware?)? If not, then I can see why the comments was tearing into the guy.
I can't speak for others, but a reason why I don't have much of an issue with the fmc, at least as far as her "fake" confession goes, is because the mc is already aware that it wasn't a real confession and that she was going along with what her friends had told her to do (in order to get used to boys if I remember right), and he has no problem with it and instead goes along with it until they eventually break up (as far as he knows at least). Now if he wasn't aware of the truth (or if we weren't aware that he knew), then sure I don't think I'd give the fmc much of a pass, regardless of how much she agonizes over it.