2000 Chrysler 300 Mechanic

The vehicles lights were left on for an extertended period of time. Battery died. Car was attempted boosting for 5 minutes with other vehicle but would not start and all dash lights would turn on but dim out after several attempts to try. Car was left sitting for 3 months and towed to a mechanic. Mechanic is telling me he has changed the alternator and battery and can not get it to start. He is claiming that it might be a car computer related problem. Or that it may require another key. I Dnt believe he did a proper diagnosed or changed the battery or alternator.