Yes and no to the overreacting. Some people wouldn't have worked it out after the sexting, you guys did, to a degree. I think for you, there needed to be more time between that incident and this newold friend but he felt there was enough time. Now that the friendship has started its too late to have the reassurance and how would you feel talks, only can have the reassurance talk. Which he is. Its good that they want to include their SO's when they meet. Good sign.

Have a talk with him that you will give this a try but you are still tender from that last incident and he needs to double down on reassuring things: no 1 on 1 meetings at bars or other dating like behaviors, that you want to get to know her too. Who knows maybe neither of them harbor secret feelings after all? That can happen. But as you know, so can the opposite, but that happened because he thought he could get away with it by being secretive.

When you two choose to reconcile after cheating you both have to realize you are starting a new relationship in a way with the weight of the old dead relationship dragging behind. Eventually that old dead relationship weathers away but until that time its better to be having both people pulling the weight together than just one.