Mitsubishi has a special tool for removing and installing the head bolts on the Endeavor. Also, the bolts have to loosened in steps and tightened in steps. Also, the head bolts have a special beveled washer that must be aligned correctly. If these are not followed, then it is likely to break the head bolts.
Below is complete sequence to remove and install head bolts on your vehicle:
Click here to view and download(pdf file)> head bolts
With this information you can quiz your mechanic and find out if he followed Mitsubishi's directions. If he didnt, then he is likely at fault for the broken head bolts. If he did follow their directions to the letter, then it is just an unfortunate co-incidence.