Facebook Messenger texts and my husband

An old girlfriend contacted my husband on facebook yesterday. (And by old I mean they dated like 17 years ago) He told me about it and even *kinda* told me what she messaged him. I didn't have a problem with it at all, I feel secure...or at least I did. He has his phone, iPad and our home computer all connected so any message notifications come up on all devices. I was using the computer yesterday and a notification came up that he had a message from her, three hours after I assumed they stopped their conversation. They continually chatted for several hours all morning! Nothing too bad I guess, more than he told me for sure and more than I was comfortable with but I let it go. Basically she was sorry for how their relationship ended, blah blah, then she said how much she loved him and still thinks about him often. He responded with how much he cares and appreciates the apology and he was too sympathetic to her feelings that came 17 years too late for my liking! He also kept sending smilie faces and cute crap like that. Whatever, I still blew it off. Today, he adds his cell number to Facebook out of nowhere after having Facebook for several years. Also, while on the computer today, I see he has a text from Facebook for a 'Facebook messenger confirmation code' He has deleted this message. I googled it and I'm only more confused. What is it and why is he getting it? Oh, no further messages with his ex, but I have a bad feeling about this confirmation code and th fact that he added his cell to FB
Edit: he already had messenger on all the devices. Idk if he was using his iPad or phone when they started messaging. The messages went from around 9:45 am to about 1:15 pm yesterday and as far as I can see on his messenger, conversation has stopped
* He came home from work and hanged to go to our new house and do some work (we just bought and are fixing it up before we move in) She lives 2-3 hours away so I'm not worried about them meeting up but I'm crazy paranoid he's having a 'secret conversation' on messenger with her. In that case, it's only on his phone (the iPad is here) and it will delete after a short period of time. What if he tries telling me they stopped talking yesterday? If they're having a secret convo, he can lie because it'll be deleted. I'm driving myself crazy!!