Okay, no rain this morning. Back to research.
As I have no way of knowing just where the filler tube was disconnected I'll give you both options.
The first applies if the tube was disconnected from the body with it still attached to the housing (#13 in my link). That housing is attached to the body with two machine screws, part number 15966436 (#14 in the link), and one plastic push-pin retainer, part number 11561878 (#16 in the link). The two screws are round head, Torx type, 6mmx1.0x24mm long. The push-pin retainer is for an 8mm hole, panel thickness 3-7mm. Retainer head diameter is 15mm. The specs are in case you can't find them by part number and need to substitute.
If the filler tube was disconnected from the housing, you will need 2 each of part number 11519940, item #12 in the link. I don't have the spec on those. They are two smaller sized sheet metal hex head screws (7mm hex head), about 20mm in length. Depending on what was done during the body work, you may also need item #17, part number 11513599, which are the clip nuts for item #12.
It would save some time/expense if you could get your father-in-law to let you know if the plastic housing is what is moving around or just the filler tube. Another point of concern is the ground wire for the filler tube (#20 in the link). On my Crew Cab I can determine if it is secure by looking from under the truck. It attaches to the frame.
Hope this helps. Good luck.