When they did the belt did they put new all new pullies ect on or at least check them. It is recomended that they do so. Check the bearings in the pulley to see if they seized if not maybe mechanic didnt torque to the right spec. I was wondering why subaru didn't recomend putting some loctite on the threads before reinstalling so something like that would not happen. Im going to do it to mine when I do it.
Either way Old pulley bearing seized or not torqued to spec Id make him responsible for a New engine!!!!!