I'm sorry, too, that you are having to deal with this. Friendships occur quickly when people are in the "trenches" dealing with life and death issues (like recovery). New relationships don't carry the baggage that established relationships have to them.
As hard as it is the only thing you can really do is turn it over and then work on your self. You can't control whether she has a friendship with him but you can determine if you want to react and how much.
It's just hard and I'm sorry for all that you are experiencing.