As noted above your companion's successive flights will be cancelled if they miss one in the middle of the itinerary without very good reason.
If they are so strongly Anglophilic, they could avail themselves of a flight to the UK and then a flight back. They can go back-to-back at AMS, so they buy a separate ticket AMS-LHR (not necessarily on BA, Easyjet works fine on this route) and also a BA ticket LHR-AMS on the same aircraft that they are already booked to begin AMS-LHR-JFK.