Possable, but not too likely. Valve seats are pressed in pretty good. Did you Ford Mechanic friend put your car on an ignition scope to check it out or is he just guessing at this point. What exactly is the code and the description for it?? Does it specify a lean misfire or ritch misfire? also have you tried doing a compession test or cylinder leak-down test?? One of those will tell you right away if there is a loose valve seat. Ask him what made him come to that conclution. Because if he just guessed loose valve seat I want to make sure I never bring my car to him, because that would be a hella exsensive repair, not to mention if he got the head off and noticed the seat was ok.