[Avodah] Are there limits on what must be returned

[David Riceman]

RYN (cited by RGD):
>>    Rabbi: No need to return lost smartphone
>>
>> *Halachic ruling issued by Rabbi Karelitz says because advanced cellular phone is 'not kosher,' there is no obligation to give it back to its owner*
>>
>>
It regularly happens in the gemara that some student sees a guy doing 
something wrong, complains to his rebbe, and the rebbe says "hanach 
leih, k'rabbeih sevirah leih."  In this case does RK think that his 
opinion is the only legitimate one, or (which I suspect is more likely) 
is he talking about the case of a community in which the rov accept him 
as posek, or in which the owner of this particular cellphone accepts him 
as posek?

David Riceman