Heather has been encrypted as Jgcvjgt. This would be deciphered as:
J = H, G = E, C = A, V = T, T = R
Therefore we can deduce that TQDGTV, from the letters we have already deciphered, will be ROBERT. We can make an educated guess what Q and D are two reasons, one; that the name looks like it is Robert as we know this to be a real name and two; if C = A it would make sense that D = B, and if T = R then Q = O.
c d e f g (cipher) = a b c d e
q r s t u v (cipher) = o p q r s t