Is he cheating? If so now what?

A few years ago a friend of my husbands came to stay with us, he is older, I am 29 about 27 at the time my husband is 35 and he (Mark) was in his fifties, but he needed some time to get on his feet. During his stay we wanted to do something nice for him, and he had a "friend" who lived in OH who is married but he wanted to visit her and she wanted to see him too. We drove to OH he, my husband, and myself. She did turn out to be a great person and we had a great time together. She even called me last Valentines Day to go out and get Mark a gift from her. Since then, and for almost a yr Mark has moved out, and we no longer have any contact with him. I noticed on my husbands cell phone bill he still texts/talks to her several times a day. At first my husband told me they were just talking about Mark, and she was just trying to track him down to see how he was doing, so I let it go. But the texts have continued and are very frequent. I never check my husbands phone but just on a whim, I scrolled thru his texts and there was one from her that was terribly explicit talking about the way he felt inside her and much more.... I have not said anything to him, for crying out loud she is 6 hrs away (one way)!!! not to mention the fact she is 49 yrs old!!!! I mean she looks good and all but COME ON??!!! What do I do, how do I react. I guess I should mention I have a 2 1/2 yr old and a new little boy who is only 7 weeks. I love him, and he seems to love me, what am I missing, how do i proceed? I don't want to throw my family away but at the same time how do I live with such betrayal? Please help, I don't want to talk to my family and friends since they aren't always unbiased and they seem to judge more than advise. Thanks so much