My husband has recently become friends with a woman that he met while on a trip. She is also friends with many of our mutual friends and came to visit. Apparently they stayed up very late one night talking.
He hasn’t told me about it. Also I’ve discovered some flirty messages between them. He told her she was beautiful and he misses her. I’m not terribly worried because she’s not around here right now. But I know she plans on visiting again. Should I let the crush run its course?
LemonVibe 
Have you considered a open marriage or polyamory?
If this crush doesn’t bother you then you should look into it.
If it does bother you, then talk to him about what you have found and start visiting a marriage counselor.