That sort of rust would have been accumulated over time. Won't only be from that guy but yes he shouldn't have done it like that. I wreck these cars all the time and they are actually a very common thing where people assume that is the only jacking point and jacks it up there when that point is only really for the little scissor jacks with the indent for the rail to slide into locking position.
I usually jack it up from the chassis rails, though they are very soft and should only be done in a certain way at the hardest point, usually where the two pieces join together. Though because I wreck these cars, I don't give a shit where it gets jacked up since the shells are to be scrapped anyway, unless its a straight and good one.
Best way to stop the cancer is to get a wire brush, saftey glasses, mask and scrub it off with the brush, spray degreaser over it to get the rust dust off and spray some rust guard etch primer over it to stop it from rusting again. Rust Guard primer is available at Bunnings in their paint section.
Though there may be better ways to go about doing it, but that would be the cheapest and quickets way.