I do have to wonder why your friend didn't get him off the highway when she saw him. Thank God he didn't get hurt. Poor baby, your a Yorkie Angel for saving him.
I would go and take care of him as if he was yours,
because it sounds like he is. You put up signs, did your best
to find his family. To be on the safe side you took him to the vet,
and you are giving him the very best of care. Who ever lost him
should be very thankful his in such good hands. If they really wanted him back
they would be searching for him. Not that I would let my babies out without me,
but to be on the safe side they all have a chip. San Dee is not chipped, and no one is looking for him. So you now have a furbaby to love, and I'm sure he loves you too. I don't think your wrong in any way, he's your baby, and your doing what is best for him. Enjoy.