Thanks very much for your advice, guys. Valuable for the future, for sure. (Am replying on tom's behalf, he's getting ready for sleep & his ride onwards towards Montréal tomorrow.) We checked the amount of chain stretch, and it's still less than .50, so all OK there. Also checked the amount of slack in the chain, using Thorn's guidelines, and Kathy's chain is well within Thorn's recommended limits. Pretty sure that they'll be OK to get to Montreal, and then to NYC.
Cheers, John