I'm not keen on loaning tools and have only done it on rare occasions. And certain tools are completely off limits - (spray guns, torque wrenches, etc., come to mind).
In your case, loaning a buffer, I would have told the user they need to buy their own pads, or pay for replacements. Pads are expendables - even if used and cleaned properly they only have so much life in them.
I've found the best way to clean foam pads is to simply run them under a faucet (water as hot as your hands can take), scrub with your hand, wring out... then repeat as necessary until they are clean. Once clean I put them on the polisher at high speed to sling out the remaining water. Then let them dry overnight or longer before sealing them in zip-lock bags.
If the pads have been used for wax or very oily polishes I then add a little detergent to the process to break down the wax/oil. Tide and/or Dawn, or the like, have always worked for me.
