The very best thing you can do for that bike, whether it's for you if you keep it, or your friend if he gets his affairs sorted out and buys it back, AND for the bike ....
Is to Ride It! Ride it enough to have to change the oil every three months or so to keep things fresh (no filter on that), and to prevent cobwebs and spiders and rust and ethanol-residue and all that stuff!
Lannis