Most likely, the "borrower" refueled with a less than desirable mix ratio. I worked at a mower shop for many years in a past life and this was the #2 "it won't run" issue (#1 was being out of gas)
Start from the beginning. Dump the fuel, run it dry, pull and clean or replace the plug (really it's only a $ or 2), refill with proper mix and give it a shot.
First thing we used to ask customers on the phone before we charged $50 for a pickup is "Does it have gas?"
In this case, does it have the proper gas?
Always eliminate the simple, and easily fixed problems, first.
