You should get to know this girl and be her best friend too. She’s probably a perfectly lovely person and all of you will have a great time together.
Gorgeous women like that have a lot of problems. I’ve interacted with more than my share. They have a hard time finding guy friends who aren’t really pig-headed and just turn every interaction into trying to sleep with her.
I’m gonna level with you, your boyfriend would probably be with her if it worked out that way. But some confluence of weird events made that impossible, so he had to move on and seek greener pastures. You are those greener pastures.
You should feel lucky he even told you about her. I have a phenomenally gorgeous female friend that I can’t be with that I will absolutely not tell girls I’m dating about until I’m damn sure I can trust them.
He very probably knows exactly how you were going to take the news, and decided to tell you anyway because double lives are ugly and miserable and way way worse than just leveling about it. That he’s telling you is a sign of how strong he considers your relationship to be.