You're in a tough spot. If you can turn a wrench I would say do the work yourself. I often do as don't trust professional mechanics. I know it's unfair of me but over the years I have come to my own conclusions. Honestly I would not pay myself for a repair as I would take too long...
Pay for it and pull the head your self and check the valves. Then sue them to get your money back. OR get a lawyer and sue them upfront to reach a settlement. I bet they did not tighten/replace the idler pulley causing the timing belt to fail.