Got a serious questions for everyone!!!?

My boyfriend of 2yrs has been talking to his ex-wife of 8 yrs on the phone. Mostly texts and the one I've read was completely platonic. She has been diagnosed with cancer and going through a rough time obviously. The only problem I have with this is that he kept this from me and wasn't up front about it. They were going to make dinner plans according to these texts and I was completely unaware of this. When I confronted him about it he said he was trying to not get me upset because he thought I would have gotten pissed. I asked him why he was doing this because she treated him very badly and he said because he feels empathy for her. She has a son (not biologically his) and is going through hell and back. I asked him to be honest with me and let me know what is going on from now on with this situation and he agreed he would. What do you guys think? I feel terrible for her and what she's going through but is it ok for her to contact him for support even though he's in a relationship???