On the one occasion (many years ago) that I had to leave a rented aircraft at another airfield following a diversion, I made it my business to gget it back to the home airfield at the earliest opportunity after the weather cleared the next day. It never occurred to me that it would be anyone else's responsibility. If the renter had had to arrange the return I would have expected to have to pay at least some of the cost - although I would expect that I would have been asked to arrange the return in the first instance. There are lots of risks entailed in flying - weather diversions and associated costs and inconvenience are amongst them!
I've mostly flown syndicate aircraft since then. Our current syndicate agreement says:Members are expected to take all reasonable steps to return the aircraft to base at the end of any booking period. Where this is not done, the other members will reserve the right to take any necessary steps to arrange return and / or storage. The member concerned will be responsible for all costs associated with the return unless the failure to return the plane was a result of mechanical failure.
There is then a paragraph about mechanical failure which still puts the onus on the member to try to sort out a repair and return the plane if that can be done within a reasonable time frame. Even if other members of the group ultimately have to return the aircraft to base, the member suffering the breakdown is expected to pay for that flight.