i just have to take his word for it....
Or you could pay attention to the fact that he didn't do a thing to re-assure you that she is out of the picture but instead turned it all around on you until you ran away from the subject like desperate chickeeeta. Do you think you could simply ask him to clarify why she would think that he would appreciate a card such as the one she sent him? Why did he break up with her?
Anyway. If he's doing her then don't you think there would be other signs that he is? What actions (if any) are you also ignoring that indicate to you that he's being sketchy?
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion