Check the fuel lines first.
There are a lot of good suggestions earlier in this thread and the problem could be most any one of them. Like what was said earlier, without knowing what the mechanic did pretty much any of the suggestions could be the cause of the problem.
With regard to the fuel line, that was my thought way in the beginning. The fuel return line is the "low hanging fruit" check: no cost and if it's the problem then it's a cheap fix. This is something almost anyone can do. Pretty much everything else you're looking at will be expensive and, this time of year, require a difficult to find and technically capable mechanic.
1. At the top of the injectors is a banjo fitting that is connected by a metal tube, and the rear injector's banjo is where the fuel return line is connected and secured with a clamp. If the clamp was removed then later reinstalled but aft of the barbed fitting (Note: can be difficult to get the hose on) it will have constricted the hose at that point and prevent fuel being returned to the tank. The build up in fuel pressure here will force fuel into the injector and, hence, into the cylinder. Hmmm.... this gives me another thought, see par. #4
2. The next thing to look at is the path of the fuel return line to see if there is a kink, or, heaven forbid, a shutoff valve where it attached to the tank. (a) There shouldn't be one but who knows, and it could be closed. (b) Maybe someone removed the hoses on the tank then reconnected them in reverse? [Edit: the latter not likely because the fuel tank connection for the return line should not have a pickup tube.]
3. If the hose is old there could be a coating on it's inside, same kinda stuff as on the inside of an old fuel tank. Solution is it replace the hose.
4. Later in this thread it was mentioned about one cylinder running. Maybe something was wrong with the installation of the banjo fitting that would have plugged one of the holes where the return fuel goes? Not sure how this could happen but if the bolt with it's hole and recessed area doesn't align with the banjo the fuel won't flow. All I can say here is that *stuff* can happen.
5. One other item not mentioned in the thread's laundry list of possibilities is a leaky fuel pump diaphragm. A hole in the diaphragm will cause fuel to get into the oil pan, dilute the oil, and then bypass the rings to get burnt. Doubt this is likely because the fuel pump wasn't touched. If not the fuel line, most likely the problem was caused by something that was touched when things were "fixed". Air filter: is it still there or did it get ingested?