Kegs gone Wild

So I lent a friend a corny keg so he could keg and serve a batch of beer. He returned the keg with maybe a half gallon- gallon of beer left in it which smells sour and tastes tart. What should I do to put this keg back in my regular lineup without messing up future batches, or worse, my draft lines?

Thanks guys!

Clean well and sanitize, making sure to completely disassemble connectors, overpressure valves, dip tubes, etc. If you're really paranoid replace all o-rings.

Also, don't use bleach. I prefer a hot and strong solution of PBW for all parts. Completely disassemble everything as Joel said. Wash and sanitize.

I think a lot of people leave beer in their kegs for a long time. It's best to empty and clean immediately, but I know a lot of people that just put the dirty keg in a corner and leave it until they have a few to wash. If you clean, sanitize, fill and keep at refrigerator temp, it will be fine.