advising someone to comply with the law or a bye law is probably a civic or ethical duty.
advising the person a reason for the law or bye law 'its disgusting' is possibly inflammatory', but she probably wouldnt understand a medical or legal argument, so its not an unreasonable position, and may be seen as a concise summary.
Spitting on someone in any time is an offence in many sensible jurisdictions. He should have of course called the police and have her arrested, but no judge is going to prosecute him for taking the next most sensible action, which is remove her from his vicintiy. the bus wasnt moving so she wasnt recklessly endangered at all.
In a pandemic, spitting is clearly a higher order, he could probably sue her for plenty.
I support anyone's right to ave a pro or anti mask position, but not to break the law.