MY husband and his female friend

My husband i were happy until we had a child . my husband has been emotionally attached(every single text had kiss emojis) to one of his female friend even before our marriage. they have been in constant touch even though she is in other state now. I told him I don't like the way they speak and to stop it and he refused. my husband threatening to divorce me if i speak to her regarding this, They are still talking she is married now. He is planning a secret vacation with her (lied to me as going with his guy friends). I want to know few things, a) is it illegal to go through my husbands phone b) is it illegal for me to contact her c) is it legal for them to continue this so called friendship even if it my emotional distress how can i make my life better without getting a divorce and taking her out our lives.