No need to get mad at the guy who dared him to do it, he's not your boyfriend, and shouldn't have a part to play in your relationship. It was immature, yes, but although he ancouraged it, he does no have control over what your boyfriend does. Your boyfriend didn't have to act on the dare, he could have been responsible and 発言しました no, regardless of what his フレンズ would think.
Whether you're mad him または not, and are going to take it seriously, is up to you.
First find out if he was actually dared. People use the excuse 'Someone dared me to!' a lot. I'm sorry- but maybe your bf actually likes that girl. I'm sure if he was loyal and liked あなた a lot, he wouldn't accept dares like that.
Well, just don't do anything stupid and あなた should be fine.
And yes, stupid includes blaming someone before あなた talk.
Try talking to your boyfriend and see who dared him. Then talk to that person. Then あなた should talk to the girl, if あなた need to after that. Once あなた get your evidence together, either be mad at the darer if it really was a dare (because he probably knew あなた were his girlfriend in the first place, または your boyfriend 発言しました something but was pressured into it. Boys are もっと見る prone to peer pressure than girls because, whether they want to hear it または not, their popularity is just as important to them as it is to us girls. Plus they really don't like being called chicken または weak), または if あなた confront who your boyfriend says dared him and he says he didn't dare him after an extensive period of making sure he's telling the truth, then あなた have right to be mad at your boyfriend for the note issue and also for lying about it :)