Melted fuel pump assembly 2006 yaris HB

[porkchop]

Car wont fuel without regurg up the filling neck. Checked everything and finally pulled the cover off and exposed the top plate of the fuel pump assembly. To my shock, the vent hose, vent hose port and ring lock were completely melted. (explains why the tank was not venting!). The fuel line and electrical for the fuel pump assy which are only centimeters away were spared, amazing the car didn't burst into flames. No idea what could have caused it, it's not my car it's my sisters and she says it just one day wouldn't fuel properly. she had some exhaust and other work recently and the rather shady mechanic she went to said he couldn't get some bolts out due to rust, I wonder if he applied a lot of heat to the floor pan which travelled to the little hatch area where the fuel pump assembly is accessed. It would have had to get pretty hot in there to melt plastic like it did. Kind of concerning since I don't really know what caused it. Any ideas would be greatly appreciated. I will attach some pics at some point.