Ex coming onto my best friend.

Ok. What it says in the title really. They have never met but each thinks the other is hot, but recently on facebook he has started commenting all my pics of her saying how sexy she is and has added her as a friend and stuff. I'm completely over my ex, but our break up was a mutual decision, he was my first love and i guess part of me will always love him. We are great friends with no messy relationship baggage left over and i really do want him to get a new girlfriend, just not my best friend. I kept saying he should come and visit me at college and he was always like 'no i can't, i don't have time, i can't afford it' until he saw a pic of my best friend and now he's like 'yeah maybe i should come down'. Its really getting to me, not just that my ex likes my best friend but it has happened with other people too, they'll fancy me for a bit them they'll see a pic of my best friend and i don't exist anymore. Its even worse it being my ex, we were together for a year and a half and though we definately don't want each other back, i don't think i could cope with him getting with my best friend especially since i live with her. Imagine what that would be like. Should i talk to him or would he think i was just getting in the way and start accusing me of wanting him back?