I’m surprised no one has more concern about the sheep getting the kind of care it needs from an owner who is knowledgeable about sheep. A new owner can educate themselves, but it needs to be done.
If the sheep is a stray, it is the property owner’s responsibility to remove it. If the HO wants to assume ownership, I assume that all responsibility for care comes with it … and that the sheep could change the nature of the boarding arrangement since it is set up for horses, not sheep. If the sheep does damage or causes disruption to others, or if other boarders complain, who is responsible?
Who legally owns the sheep now? Does it have any monetary value? (Maybe not since the original owner doesn’t seem to be looking for it.)
There are consequences to every decision … and to those decisions that are never made, as well.