Is it okay for my husband to have pictures of a female work associate?

My husband of 14 years has been spending a great deal of time away from the house. He's been working loads of overtime, spending time with male friends, and helping his mom when she needs him. The extra money he is making has not been coming into the family bank account, instead he's been paying off his own bills. In addition he openly flerted with a friend of a friend. This gave me reason to suspect something might be going on, so I started snooping. What I found was a picture, in an interoffice envelop, or another woman. When I confronted him, he stated that this woman was a work associate from 4 years ago. That she had come on to him and given him her picture. He had never pursued a relationship so he says, but had kept her picture. I think there is more to this story but I have no other evidence.