Can chemotherapy cause a person to lose it mentally?

I have a friend who is about seven or maybe five years into remission for leukemia. He had chemotherapy for three years. This person is a college professor and I have known him for many years. Last winter we were at a rock concert and I heard him telling people that he knew the singer personally and traveled with the band. I know this isn’t true. I lived with him as a rent paying room mate for a few months, his behaviour is so odd and irrational that I had to leave. In a hurry. I don’t remember him being crazy years ago. Could it be the chemo?