I think it would be better to err on the side of caution and give your friend the benefit of the doubt before you assume he's lying. Finding out you have cancer is a huge shock, and people react differently to the news. He is just now beginning to find out more about the cancer, the medications used to treat it, etc. He may not be able to remember details yet.
If you do eventually find out he's lying, then he must really have some problems upstairs (in his head) because that would be a pretty cruel joke. I wouldn't want to be tested like that, someone to say something like that to me just to see what I might say or do. No way! But don't jump to conclusions...wait and see what happens. Maybe you could even offer to go along with him to one of his appointments or something. I think it is best to assume he's telling the truth until you find out otherwise.
Nancy