Perhaps one possible solution would be for the cat to return to jo, but for his new family to visit from time to time. I can't remember how long jo had had the cat, but I think it was considerably longer than a couple of weeks.
For me love for a cat grows, it isn't just that instant aaahhhh we all feel. The habits ingrained by the daily caring for a cat: it's feeding, sleeping, grooming and play routine create a strong bond which is greatly missed when the cat isn't with us, for whatever reason.
The RSPCA are undoubtedl the villains here, but now that they have committed the crime, the consequences have to be dealt with.
Good luck jo.