break up. no need to give an explanation just tell him he’s overrreacting. literally please trust me and break up you really don’t wanna go through all the useless drama.

OP, you kind of have three choices: (1) Ignore that boyfriend might be stepping out with his best female friend, (2) Get into a fight about it and risk a breakup, or (3) Break up with him... I agree that it isn't necessary to give a reason for a breakup, but we all know that you will be telling your former mutual friends something when they ask about the breakup. So you might as well just say it was because you just couldn't trust him. It's not necessary to "prove" anything is going on to breakup - Lack of trust is enough. Of course he will claim that you are overreacting - Even if you had caught him and his "best friend" in bed together. OP, If you not willing to skip ahead to breakup, then the choice is either pretending this isn't an issue or get into a heart to heart discussion where there is a good chance that he's going to drop you instead of his "best friend"... I believe it's better for couples to work out their differences in an honest and non-accusatory way when they can. The exception is when it seems that there is a serious lack of trust. When that is the case, there isn't much of a relationship. Either one partner has serious insecurities which shouldn't have to be carried by the other or the other partner has a pattern of apparent lies and deception which shouldn't be tolerated in an exclusive relationship...

that’s his woman not his best friend. you may want to leave that situation

Dump him. He should be doing cute relationship stuff with you, not with her. That's a huge red flag.

I’m guessing he hid the pics where they’re not wearing any pyjamas at all. Dump him.

I hate to tell you, but you're the other woman. it's time for him to go

EX boyfriend. You deserve to be with someone who is honest with you (and themself).