An' naebody kens that he lies there, o
But his hawk and his hound and his lady fair, o.


Given the context - the terse, grim, sardonic dialogue of the twa corbies - I don't see how you could interpret those lines in any other way than to mean that the 'lady fair' had a hand in the death. Why on earth else would the lady fair not happen to mention to anyone where the body is? That is a rhetorical question; please treat it as such.