Did the temperature gauge make it all the way to the red? Subarus are, in fact, known for head gasket problems, and the leaks can present in multiple ways - not necessarily via poor running or coolant in oil, etc. It depends on the nature of the leak. But one thing that can get a head gasket to leaking is overheating. So its possible that the shop messed up on refilling w/ coolant. Thus the car overheated, thus the head gasket blew.
But its possible that it was already leaking too. Do some internet searching on Subaru head gasket issues.
Stop taking your cars to fast lube places.