By the sounds of it I would say there's something wrong with the conditions your mate keeps his ferrets in..
As Ideation says there are issues regarding the Jill jab, as well as what he wrote I've also heard of them getting tumours after getting the jab so is that really an option?
You asked wether anyone did an experiment and i told you an example of what happened to a friend he obviously he didnt do it out of spite or deliberately as an experimant probly more to do with ignorance/not knowing theres also an implant ive heard about for jills and yes they will come in season twice usea