If that car is kept in a garage, the mudded up repair would last forever... how do I know? The old school body man who painted my Nova 20 years ago fixed a big dent in the door the exact same way. The filler never fell out or cracked and the backside never rusted. Not saying its the "right" way, just sayin it can work. The guy has a right to fix his car however he wants...but I think he HAD to know his method would be frowned upon here!! I think he's pushing your buttons guys!!!
What he's doing is a little different than stuffing newspaper in a rusted out dog leg and mudding over!!