Don't want to dampen your spirits, but if it drove 250 miles without a problem after the HG change, there is a good chance it's not something the mechanic has done, but the failure of a component after the HG change, maybe as another member said the oil pump had a hard time with the old sludge, and was on it's last legs, and died on you after the 250 miles.
If you have driven on an engine with no oil being pumped, then the outlook is not good. When this happened on my GLS the con-rod came through the block and ruined the engine.
Unfortunately I think your recourse on this mechanic is nil, and I hate to say, maybe rightly so. If you've driven 250 miles with no misfire or coolant/oil loss, there's no major faults with his HG change.
Just an opinion. Really sorry this has happened at this time of year, I feel for you.