Fysa, they almost certainly completed the rescue using their twin 500hp diesels rather than sail power. One sailboat towing another creates a host of sail/wind/wave issues. Almost every large sailing vessel, including historic replicas, has diesel engines for those times that sail is just too awkward.
In this case I am sure you are right, and the size difference would make towing under sail dangerous. A sudden gust enough to accelerate the larger boat to above the smaller boats hull speed could pull the smaller boat underwater quickly.
Most dangerous IMHO is a wave that slows the large round hull but will not slow the smaller wave-piercing hull, sending it slamming into the stern.
You can see the distances they chose in this particular case in the third image from their Instagram post.
https://www.instagram.com/p/CryES46tgWl/?igshid=NTc4MTIwNjQ2...
Unless I were to start using halyards, I probably do not have more than 300' of line on my boat, which is really way more than I need for a smaller boat.
Also, the road taken is interested. They could have go to one of the ports of the British islands, like St. Peter Port at Guernsey. But this would have been dangerous. This part of the English Chanel, is full of smalls rock islands, shoals and one of the most dangerous marine currents in the world : The Alderney Race (or Raz Blanchard in french).