I agree with the majority of these posts but I also find the blind bashing of this man a bit ridiculous, also.
He said he forgot, but what if that wasn't the full story? What if he is going through something else at home that is on his mind? Maybe he has just lost someone or similar. He didn't need to disclose private information to a stranger on the bus. Or, is it because he's a man we just assume he's a pig who couldn't be arsed that day?
Secondly, no matter what illness you have, you cannot go off at people for not wearing a mask. She had absolutely no idea why he wasn't wearing one, and although OP stated he usually wears one, she wasn't aware of that. One illness doesn't trump another and she should have kept to herself or got on another bus.
The only entirely safe way to travel is via your own car. If that isn't possible, then unfortunately public transport comes with it's own risk. There will always be people who don't want to wear a mask (whether you agree with it or not). Similarly, there will always be people who cannot wear a mask due to medical reasons and they should not have to justify this to a random person.
We can only control what we ourselves do. It's an awful situation for the woman. I can't imagine the anxiety she feels when going out. But, unfortunately, there will always be risk, and going off at someone is unacceptable.