You should have done this for your customers years ago, yikes.

I suspect that he/she means they runs a mail server and it was up to his customers to comply, but now they’ve had to deal with the fallout from their customers not complying.

Cool, charge them for it. That's what IT support is.

The customers have to do it themselves since they control their domains. I recommend doing everything correctly but also understand that this stuff is pretty hard for people just trying to run a small business. But if it makes you happy to feel smug you can carry on assuming.

My assumption is that they were in control of the domains. If not, it isn't their problem and if anything it's billable hours, aka their job.