1. replace the heater core, problem solved
2. get a 2 lb cap and container of alumaseal or copperseal if you can find it. Dump it in , change the cap and start looking for a new heater core,
3. find a non pressure cap or just leave this cap loose, on the first stop. drive it until the weather heats up enough to boil over. (unless he has a super clean cooling system)
Then replace the heater core.
