Doc left my town a long time ago but hes in my town square everyday and none of my friends have him so I think he can still be adopted. I want any wolf or any penguin for him, or a villager in the "Very popular" section.
People can't adopt villagers if they've moved out of your town unfortunately. The only window in which you can ask a villager to move in to your town is when they were in boxes. He's not in your friend's towns because he's in "the void" which means random towns anywhere.