If they're in gold-only seats, like the exit row, they will likely get moved. If they are in ordinary seats next to each other, I expect this will not be discovered.

There is not, as far as I can tell, some sort of seat-disorganiser to pessimise seating for people who haven't paid. This isn't Ryanair you're flying. There is only a check, every so often, that people sitting in exit rows or row 1 have either paid, or are Gold(-equivalent), or associated with Gold(-equivalent) person and they get moved to a random other seat if they're not.

If you cancel close to departure, this probably won't be discovered in time and they'll probably keep their seats even if they lack the Gold(-equivalent) association.