Any 3.5l gurus out there?

Looking for ideas/suggestions here. I can get a 1996 Concorde for FREE! The car was my parents before my niece bought it. The rub is: she overheated it to the point it stalled on her. The car re-started later and she has been driving it for several months now. However, it is low on power, so it was checked out by a competent mechanic. He determined that 5 cylinders have 120psi of compression, while 1 has 80psi. My question is this-blown head gasket, warped head, or just bad motor? Hate to junk the car because the body is really nice and has low miles.