Has that
always been the case?
As I said, it is remotely possible that a random mutation occurred, and your friend wound up with a freakishly rare event in which a previous form of a gene got recreated. It is far more likely that a normal coated rabbit was involved, and someone is misrepresenting the parentage. However they came to be, they technically are not
any kind of Rex, Mini or otherwise, since they don't have Rex coats, so your friend
is misrepresenting them now by referring to them as Rex. It's kind of like my horse Syd. Both of her parents are registered Miniature Horses, but Syd is 40 or so inches tall, and every Miniature Horse registry has a height limit of 38 inches or less. Pedigree alone does not make an animal a Miniature Horse, it must meet the height requirements too, so Syd is not a Miniature Horse. Those of us that have these oversized animals may call them "oversized minis" or "unregistered Shetlands" or whatever, but all they really are is small, refined ponies.
Another example might be a line of Harlequins that I had. When I bred two specific individuals together, I had REW babies show up in the nest box. I had bred these animals for a half-dozen generations, and I knew I hadn't done any outcrosses. These two rabbits had a common ancestor a few generations back; she had a grandparent that had come from the rabbitry of someone that did an outcross to a NZW to improve type. This REW gene had lurked, hidden, for several generations until I happened to cross two animals that both carried it together. I jokingly referred to the REW's as "paint-by-number" Harlequins, but technically, they weren't Harlies at all without the markings. The difference here, is that the normal coated gene is dominant, not recessive, so it
can't lurk unseen.
Your friend may have some small rabbits, but unless they have the Rex coats, they aren't Mini Rex.