Why? How? Where? When?

We have been together 2 years. Things have always been great I have never suspected he would ever cheat. About 4 months ago we had a miscarriage. Since then things had gotten very hard. We began to bicker and argue on more frequent basis although, we always seemed to work it out.

Anyways, about a month ago I had been talking to a girl whom i used to work with. This girl would constantly message me telling me a lot of personal things. One thing i found she would constantly bring up was that she had been talking to a guy who had a girlfriend. She explained that he was saying a lot of innapropriate things and felt that it was unfare to the girl but that she wanted to continue talking to him. At first i thought nothing of it.. but later on i had noticed she was using the name 'tyler' which ironicly happens to be my fiiances name. Tyler is a common name so instead of freaking out i tried to keep myself calm. I confronted him about it. I didn't accuse I just figured I would ask .. He stayed very calm while telling me he would never do that. That he loved me and only wanted to be with me.

Two weeks later the girl e-mailed me and asked me to go for coffee as she had something important to talk to me about. I learned then that he had been talking to her for roughly 2 months. He had been talking to her by text messaging. He memorized her phone number and would delete every message made.. and he messaged her every single day. these messages were filthy and disgusting... how can someone do that when they are committed? If you are not happy simply leave instead of torture the person you are with.

Anyways after talking to him about it as well as her.. i learned nothing had happened. Or so they told me. They both said that it was all talk about having sex. Which still does not make it rite. but i still decided that he deserved a seccond chance. now it has been 4 weeks since i caught him... and this girl messaged me again the other day. We began to argue as I don't like her, I thought she was my friend and she went behind my back... during our arguing she said you can tell tyler to stop calling me.

Here is what im trying to face now...
She was very unhappy that i didn't leave him, so does she want to be with him? Is he still talking to her? She said he was calling from blocked call so that he couldn't get caught and so she could not proove it to me. What do i do and how do I find out the truth?