I am new to this so please bear with me! Here is my story:

My husband of 6 years and I have been separated for almost 6 months. It was discovered by me that he was having a relationship with a co-worker who lives in another state. They would talk extensively at work via phone/work chat and have had been in the same place on business trips. He of course denies that it is nothing more than a friendship, but our marraige began to suffer and I told him that if he was not willing to cut off the relationship and work on our marriage that he would have to leave. He chose to leave. During the time he has been gone we have remained in contact and he tells me there is no longer a relationship and that he wants to work it out with me. He says he is also going to therapy to work on his own issues and would consider having me join him at therapy. I checked her FB and it says that she is "involved with a man she loves very much" I asked him if she was referring to him and he says no. I then told him I could not join him in therapy until I had proof that their relationship was over. I gave him one week to prove this. The next day I checked her FB and she has set everything on private and I no longer have access to see what is on there! What should I do now?

It sounds like they're still involved in the A. It's no coincidence that her FB page has suddenly been set to private. And of COURSE there's an A - don't let him gaslight you. No man leaves his M for a "friend".

What do you need to have to prove that the A is over?

Yes, sounds like he's still in the affair. Do you have children? Is he willing to be completely open and transparent with everything to prove to you that the A is over?