This is actually DH's AIBU.
17 years ago when DH was a uni student he gave his friend free dental treatment. DH is an extremely loyal friend and he is still good friends with all of his old school friends.
This particular friend, call him John, is a total waste of space and made no effort with DH for years so they just lost touch, and they fell out years ago because I didn't invite him to DH's surprise birthday party. Anyway, DH and John are both ushers at their mutual friend's wedding this autumn and completely randomly John messaged DH to say that he's having issues with the tooth DH treated 17 years (!) ago so he wants several thousand £ or he's going to take legal action against him. He also said that he was planning to discuss this at the wedding this autumn, so he was planning to ambush DH and cause a scene at their friend's wedding.
Now legally, John has zero case against DH because the treatment was done 17 years ago and there are other factors too, but now DH really doesn't want to go to this wedding and be forced to 'work' with
John for the whole day. He's actually worried that they'll end up getting into a fight if John tries to get into an argument with him. Their mutual friend, the groom, knows all about this and is annoyed with John, but still really wants DH to be there for him.
I can't go to the wedding because we have young DC and they haven't been invited, and the wedding is a 5 hour drive from home so we couldn't go up just for a day. I'm also breastfeeding the baby so I couldn't leave DC with anyone, not that we have family here who could look after them anyway.
So is DH BU wanting to back out, but I'm also a bit surprised that John will still be in the wedding party, even after he planned to use the wedding to ambush DH?