Originally Posted by jdrtravel

This is the most important point. If they miss their connecting flight at JFK because of anything outside of their own control (delayed inbound flight or long passport lines), Delta will rebook them at no charge if they made their best effort to catch the flight. In fact, if TXL-JFK is late, DL will probably rebook them automatically before they even land. What they obviously should not do is miss the flight because they were at an airport bar or something like that.

The bigger concern would be a repeat of what happened in Paris. They need to be at TXL on time. Sadly, it's a pretty awful airport to hang around in, especially once you clear security, but I trust they will be OK given the recent experience.