If their tank is that dirty, can they even see their fish? If not, grab a few plastic bags and when they aren't paying attention snag a fish or two until they are all some place where they will be loved and cared for. You'll have to be VEEERRRYYY sneaky about it, but if they never clean it they won't notice.
And then one day it's, OMG all they fish died and must have been eaten or sucked up by the filter
"
I did something similar with 3 Pit Bull puppies that were being seriously neglected. They went to great home and lived long happy lives.
I'm very devious when an innocent life is involved.