At best he is being niave at worst disceptive. He is not respecting you or your relationship. He is not 'free' to have dinner with another woman without you being present. If she is 'just' an old school mate and he wants to play catch up then arrange a foursome to include their partners. A private dinner is playing with fire.

I would take this as a huge warning that he is not ready for marriage. Being mad at the woman seems displaced to me. Sure I get it and I would be mad at her too but the only person you need to concern yourself with is your fiance. You can't hold the other woman accountable for behaviour your man is encouraging. Let her husband deal with her. She is out of line on so many levels but she is not the issue. He is. His lack of concern for your feelings. He is acting like a school boy and not a man due to meet you at the alter in a few months.

This would be a deal breaker for me. Recognize you are playing with fire and disrespecting me and show me you get that or the wedding is off until you do get it. I want to marry a man I can trust and who I know respects me and will not behave in ways that will hurt me. If you aren't that man then hit the road jack. I will find a way to move on.

I have men friends from school days and I exchange emails with them now and then. If they are married the rules are clear. No intimacy. No flirty and no way is there dinner or coffee get togethers without bringing the significant others along. That is just asking for trouble.

I am so sorry you are having to deal with this. I really hope your partner gets a reality check somehow and puts you first. Right now he is putting another woman ahead of you. Not good.