She’s supposed to be my friend

I have been with my husband for 9 almost 10 years and we’ve never had any real issues (cheating, flirting, lying, etc) so it was no surprise to me that tonight, a girl messaged him on fb, he laughed and showed me right then. Y’all, it was a girl that I have called my best friend since middle school.

She recently got hired at the same place he works, so he’s like her boss/ supervisor person. He already told me that she was flirting with him at work but honestly I didn’t think anything of it. She just ask him if he would want to grab dinner tomorrow since she knows I’ll be working and it’s his birthday.

He has plans with his family for his birthday, they have never hung out without me, so before I just go off, she’s definitely in the wrong here right? Like it’s probably not that’s she’s just being nice so he’s not home alone on his birthday. I’m not naive I just have a hard time with this i guess. What do I say to her, how would you go about it?

I’ll also add that we are the same age 27. She’s not really the long term relationship type. She’s usually only in a relationship for a few months and she’s bored.(I don’t judge. As long as people are happy then they should do as they wish) She did end up being with a guy for a year and a half and they have a 5 month old but they split last month.