They would have to have completely disassembled that BFP to solder directly into the threaded ports without damaging the rubber and plastic components. Even if they did not, it will have to be done now in order to heat the valve body. The solder will come out of the threads once the valve is reheated. At the very worst, rather than buy a new BFP, I would flux both the male and female threads and then screw them together while the solder is molten, and then just solder the joint closed. Finally, put the valve back together. As far as responsibility, the initial responsibility is with the incompetent plumber, but it goes up the line to whoever hired him, (i.e., the irrigation contractor), and then whoever hired that person/company, (possibly the general contractor). YOU should not have to pay, or buy, ANYTHING. If the valve was damaged for any reason, and I would suspect heating would have caused problems, that is the installer's problem, not yours.