Defenders #101. Doc is in a bad place after losing Son of Satan to the forces of Hell, and the Surfer takes him around the world in an attempt show him the fight is still worth fighting.

There are also some points in each individual's series that they were shown to be very respectful of each other.

I suppose it depends on what you mean by friends. They are both loners by nature. I'm not sure they (nor Namor or Hulk) truly have friends, beyond a few cast members in their own book. Even then it is spotty.

There is I believe a mutual respect between the two.

I also remember in Silver Surfer Requiem Doc tried very hard to find a cure for Surfer, and seemed heart broken he couldn't succeed.

I also want to say that the Infinity crossovers had them getting along well.