Ok, it's a little petty, but she has made a number of comments over the last year (to me directly when we have met, and to him) that have rubbed me up the wrong way. The last was asking him why he is in a relationship at all with me, after all he left his relationship with her so surely he must just want to be single or having something just casual? Also made comments about us spending time with our kids together. He is doing well at establishing boundaries with her, no problems on that front.
And no, I wasn't the OW, and they have been separated for 4 years now.
So, any ideas of something not over the top, but something to just say how pleased I am to have found him - we do have a great relationship and lots of shared interests - things he's rediscovered since being with me.