My fiance has a crush on his friend

I'm not sure if it's my own insecurities, or if what I'm seeing is real, but it seems like my fiance has a crush on one of his friends. He talks to her all the time, tells her he misses her, and occasionally compliments her. She has been very reclusive so has no experience romantically. He always seems to be trying to help her get a boyfriend and talks to me often about how bad he feels for her that she doesn't have a boyfriend.

I understand that crushes happen, even in long term relationships, but I don't know how to talk to him about it. He sometimes gets too flirty when texting her, and it really hurts me. I've tried bringing it up before, but we never get far because he becomes very defensive and I just stop talking because my feelings are so hurt. I can't help but keep feeling like I'm not good enough. I don't want to ask him to stop talking to her, since they are friends, but I also feel miserable knowing that she definitely likes him and he might also like her. help! how do I talk to him about my feelings? I don't want to control him, but I'm also not comfortable with how he talks to her sometimes. Any advice?