The \BA site says this If a delay or cancellation of five hours or more disrupts a connecting flight that you booked under the same booking reference (with a planned connection time of 24 hours or less) and you no longer wish to continue to your final destination, the operating carrier will provide you with a return flight to your original point of departure at the earliest opportunity as well as refund the parts of the journey you did not fly.
Prob the best bet but desperately trying to get him back to London somehow tomorrow but no rebooking options on the app the JER flights are on same booking reference