Was there a notice sent last November stating there MIGHT be a 3% SH for 2022? if so, they can certainly opt out by sending a 'nope, not gonna do it" notice.
The plan's operational error is letting him keep the extra. Or failing to amend to allow him to keep the extra. The ACP test should use the amount the plan says he's supposed to get.
There is a large difference in how the regulations define "catch-up contributions" and how everyone else frequently uses that term when payroll has a separate contribution source for catch-up contributions. It creates confusion.
Yes the penalties are steep, some have said draconian even. But they are what they are. Don't be late and if you have to be, file under DFVCP. And don't forget I'm pretty sure the penalties accrue from the original filing due date and not the extended filing due date. So if you think you are one day late on October 18 this year, I think you are closer to 79 days late as far the IRS is concerned.