Wow. This one is over the top. Seriously, this gives me the willies.
6) Call your lawyer. Like, now!
7) The manipulative b**** method: Surely there's someone on the guest list (assuming this is a real thing) that you can talk to other than your friend or the ex. I'd give them a call and ask whether they'd been invited to your house for a birthday celebration. If not, no big, how are you and the kids. If so, act all perturbed about what your ex is up to, since you're getting a divorce and all and aren't even on speaking terms. Make sure to use the words "creepy" and "weird" a lot. Mention that he seems a little "off" lately.
Okay, use #7 at your own risk.
In dealing with my narcissist ex, I eventually had to write off a lot of friends/frenemies who were more than willing, for whatever reason, to do his dirty work for him, or were so soft in their boundaries that they'd let him walk all over them and do his dirty work out of fear and neglect.