Mind my Business or say something

A couple coworkers of mine have been very flirty since she started. He's a flirty guy so I thought nothing of it. We all went out for drinks and to my place after to hang out. And I feel like they crossed the harmless flirting line. They were sitting basically on top of each other and her legs were in between his.. He's recently engaged and I know I wouldn't want my bf to like that with another women. I know it's not my place but I feel for his girl and want him to know that's not cool. Thoughts?