Can friability be negative? Friability test for coated tablet?

Ans: Yes. Uncoated tablet can give friability test negative when tablet is hygroscopic or the balance is not calibrated properly.

Friability test is not done for coated tablet. There no as such recommendation by pharmacopeia for coated tablets.
In case of hygroscopic tablets desiccants have been used or non permeable or very less water vapour permeable packaging material is used (eg.Aclar)