Originally Posted by monitor

When my connection is unprotected (as is the case with your parents), I do not hesitate to leave six hours between flights. Anything can hit the fan and DL will not be able to do anything for them if they miss the UA flight. I'm leaving nine on an unprotected connection international to international in SYD this December.

It is a completely different story than when an airline sells you the ticket through to your destination and takes the responsibility of getting you there. IMO you should advise them to take the six hours and be prepared to hang out in the lounge.