I think you're so far into the process and post-D life that meeting the bf shouldn't be as big of a deal compared to when she told you that she had a bf. I met exW's bf for coffee first one-on-one and that kinda paved the way. Gave me a chance to get first impressions and also let him know certain things. It was pretty amicable and I left feeling better.
Don't count on your exW holding her word. My exW didn't give me a heads up when her bf showed up for my kids weekend events. I took that through the lens of having zero expectations from her and her behaviour was not indicative of anything to do with me. I knew that she wanted me to meet her bf so that she could bring him around more - she is extremely impatient. She's made decisions that I wouldn't have made - introduced him to the kids very early on and vice versa. They're going full steam ahead with this blended family situation now and it's really interesting to watch it from afar. I try to not speculate but I do find it bizarre and immature that both of them are going ahead at top notch speed and it's barely been like 3 months since they got together. I am just worried about my kids if things don't work out between them as they are getting attached to his kids and him in some ways as well. Not a good process but I can't do much about it except make sure kids are good on my end.
I did feel a pang of nervousness last time I saw him and her together, but it's pretty low key now. So expect to feel something and if you need to head out to gather your thoughts and emotions, do so. But, it's not that huge especially when you've done so much detachment work. I have zero interest in getting back with her and seems the same with you, so who she chooses to be with is totally her reality and got nothing to do with who you are.
Your exW in some ways has followed a similar script to mine and so don't be surprised if her bf shows up to a kid event.
