Head gasket blown right after water pump replaced, now what?

I had a mechanic who replaced my timing belt and water pump on my 2002 Civic. Less than 2 weeks later, the car overheated. I had it towed back to him & he said it had blown a head gasket. I thought there must have been something he did wrong with the water pump, but he was insistent that these things "just happen". I don't have $ to keep on repairing after the $1900 I just paid him for his faulty work. I really don't know what to do at this point.