Uhm, let me disagree. The only two thing both still did religiously for each other and last two things they showed any true affection for each other in was attending each others sports events and leaving a present for bday more or less. It was more or less an unspoken pact and with how MC spoke about it, it mattered a lot to him at least.
So yes in principle it was her free choice same as for Mc not attending her games even if he had chosen to do that or MC's choice to read her phone even if that was also shitty. It was also a choice with foreseeable consequences especially being his biggest game and she knew that her message was, you are not even important enough anymore that fucking my BF, (which I can do anytime I want by the way and do not need to do especially on your big day, but hey it feels good to do that) is more important to me than being there for your big day, whether Mc had feelings for her or not that is not a nice message to your brother either and nothing had happened recently before that that would excuse or could have provoked Jaye from sending MC that message. Nor the need so big to be with Rapeboy that special day for MC with MC moving away to college in a few months at the latest anyway and she should be able to afford a hotel room if wanted to be away from MC for some sex when MC was not at one of his games.
Now her bringing Christian to games if was intended to make MC jealous would be bad enough, but hey he was her BF at the time and nothing Mc could say she was still there at his games at least even if he might have preferred without rapeboy. He was not in a relation with her even if he loved her, he had no rights to complain openly about it. Him walking in on that scene though on that day and we do not know if Jaye intended him to walk in on her with Christian in her room anyway to make him jealous after game, with how he felt about her and not knowing about that attempted rape (that we as players did) well how would you expect anyone to react? Happy? Supportive? Brotherly?
He reacted exactly as jealous as Jaye would have intended if she had not just survived an attempted rape attempt and needed a pair of arms around her. and no not very happy or friendly which is a normal reaction especially since he disliked Christian anyway beyond his own feelings for Jaye. Well if she had confided in him what happened she likely would have had those arms, then again that she did not after his reaction is pretty believable as well. Now he reacted though as he was intended to do. Pissed off and hurt and not happy for her or professing his undying love for her.
I said it before it was a marvelous bit of emotional manipulation by Stoya to make MC seem like the worst party, while it was Jaye which pulled the shitty move.