The Freon leaks out of my 2005 Chrysler Pacifica. The mechanic cant find a leak, andsaid its the heater core.Is this true? Need a second opinion.

You need another mechanic. Freon is the old term for AC refridgerant. If you have a leak, the system can be dye checked to find it. It very well might be at the connections close to the heat core on the firewall. But the heater core is part of the cooling system. Will he have to disconnect that to get to the AC lines? Possibly. Don't replace the heater core unless it is leaking coolant or he's already got the dash ripped apart and he is replacing it proactively.

If there is a leak it doesn't take long to find by pressure testing or using UV dyes. Take it to a shop that specializes in AC repairs. The evaporator coil is part of the heater core box in the dash, maybe your mechanic thinks that is the source of the leak, but I wouldn't go tearing in there without knowing for a fact it was leaking first.