How do I solve this?

So before my bf asked me out he asked another girl out (great friend of his) and she turned him down cuz she was in relationship at the time.
Few weeks ago she and her bf broke up with eachother. My bf seems to have been texting her more lately even caught him texting her while we were hanging out. (We only get to hang out once a week at most, not when there's a snow storm like today)
I finally told him this is making me stressed out and worried (told him I even got so drunk last night I threw up for first time). 
He had volunteered to cut ties with her, but me being me I am all too familiar with the pain of being thrown away like a tissue. So I said no to that. We were gonna discuss today but the snow storm got in the way and he stayed home cuz he's an hour away.
I'm thinking what I can do is basically tell that girl to not get too close to him and not text him on sundays (days we hang out)
But how do I put it nicely? How do I even bring it up to her?