We were friends before he started working for my company...

My best friend recently joined my company, for a year prior to that I had been complaining about a particular co-worker who has been making my life miserable. Since coming to work this co-worker has called and texted him repeatedly and is constantly asking him to lunch, weekend events, etc. They are both married. I don't think he should continue to encourage this behavior but he doesn't see any issue. This is causing me stress because my relationship with my friend has been severly impacted by his interaction with her.