Definitely a ring/piston problem. Good luck getting it covered, it's not likely going to be. I hate to say it, but there's really no way of getting around it. If he bought it from a dealer and the dealer sold it with a warranty implied, then he might have a leg to stand on. As it is, where you bought it from a private seller, Ford doesn't care how many people owned it, all they care about is, tune = SOL.
Ouch, is that really the stance they are taking on this or is this speculation? That would really bite. Better get ready to