and if the cooler was ever filled with something like kool-aid, it might impart some flavors into the wort.
plus I'm assuming this is a larger cooler that you're gonna put a ball valve on with a SS braid or a false bottom for sparging after the mash....is your relative ok with you disassembling the cooler?
I'd honestly just buy one. even a 5gal is $18-$20 and will make 5 gallon batches that push 1.060 OG (12-13 lbs of grain before it gets tricky).
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10