Thats sucks, tough situation though.

If the ferret's condition continues to decline and you cant get ahold of the owner i would just take it to the vet to have it euthanized. Youve been in touched with him and the president of your club. If you and the president both agree it needs to be done and the vet would agree it needed to be put down i dont see how he could fight you over it. Then again some people are stupid.