I'm unsure of the extent of damage, whole OBS sheet coming loose? but if replacing sections of roof sheets with fresh wood, EPDM, 8-9.5' wide, can be purchased by the running foot @ about $10/11+. Adhesive ($40-50 gal) is meant to be applied to clean wood (forgo all the priming). Likely need sealing/edge tape for seams. Still under 150.
Of course other than the work required to replace entire roof, removing EVERYTHING, new luan, but roofing materials itself can be had in complete kits, Rubber sheet, glue, caulk etc. Depending on sq ft for under 300. Just cost of repairing verse replacing...
Outlined patching? Top coating wood with bedliner unsure- only thing on the Thompsons is packaging states DO NOT use water base product on it, only oil based and thats after 30 days min drying time. I still would expect it to fail regardless of your friends success.
Lastly the 1/4" sheet seems rather thin.
