I just built a training program for the military on heat welding fabric. It's pretty dependent on the people operating the equipment and the equipment itself. The military uses it as a last resort for fabric shelters in the field. If done by an experienced person with quality equipment, it can be almost as durable as the material itself. If the welder is set too cold, it won't adhere. If it's set too warm it will melt the material or make it brittle.
The manufacturers use big, industrial welders and have people who do the welding every day for 8 hours a day. They can make some great welds.
The military guys in the field using a little heat gun have a big problem getting the welds to adhere.
I would also be concerned about any moisture that has gotten under the roofing material. Unless they dry it completely, they'll just be sealing it in and the wood will rot.
Good luck. I hit a low hanging limb with my Keystone and had to have the whole roof replaced. Fortunately they decided to replace the whole roof and not try to patch it.