I don't know if in other parts of the world it happens, but here in UK there's a bit of a joking custom that we "sign" plaster casts when people have broken arms or legs, leaving messages of goodwill and best wishes for the person's recovery. Normally, of course, you only sign the casts of people you know!
I would guess Ross says this here because he's in an awkward situation and can't think of anything else to say, and looking at the guy in the chair he is covered in casts, so he grabs at the first thing that comes into his head. It is a very silly thing to say in the situation, which is why it is funny.
Student or Learner