“Ex” husband (27m) hits on my cousin (32f)

My “ex” husband (27m) and I (25f) have been separated for about 7 months, not yet divorced. We were together 10 years, married 3 years. We also have a child together. I just found out that a few weeks ago my ex messaged my cousin (32f) stating that he would “be with her in a heartbeat if he got the chance.” I found this out, not from her but from my sibling. Please tell me if I’m wrong for thinking she should have came to me directly with this? And sooner? Life has gotten in the way of our friendship but me and this cousin used to be close, like sister close. Is it wrong for me to explain to her how it hurts that she didn’t tell me?

TL;DR; : found out today that my ex messaged my cousin, hitting on her. Am I wrong for thinking she should have come to me with this and sooner? Is it wrong to message her explaining how this hurts me?