Should I message a girl my borfriend has been talking to?

Well I found messages from a girl calling him baby and my love. I confronted him and he said it was nothing. He was only helping her through a tough situation and she misinterpreted the situation. But I saw he wrote in her tik tok that he was fascinated by her. So I know there was something more. Listen we have been having issues but my no means is it ok to talk to someone else while I’m there.

Long story short I took his phone and he has been without it for a few days. She messaged him today saying I’m glad you returned with your girlfriend but you should have been honest with me what a deception.

I really want to text her and ask her how it started and how long it was for. I feel like I need it for closure.

Should I do it? If so how do I begin that conversation?

I am level headed and not angry at her whatsoever. I just need some answers that he won’t give me.