Hi, I had a few spare nimutes today and found this marriage which may or may not be the one you're looking for, in the Town Church, St. Peter Port, Guernsey microfilmed records.
The original is in French suggesting both were of local origin:
Jean Mills, fils d'Alexandre, et Henriette Le Tissier, fille de Daniel, tous les deux de cette paroisse, ont été mariés ensemble le 21 Juin 1835.
Translation below
Jean (french version of John) Mills, son of Alexandre (Alexander), and Henriette Le Tissier, daughter of Daniel, both of this parish (i.e. St Peter Port), were married on 21 June 1835.
I took a close look at this as it was the only obvious Mills marriage and I'm sure it says Henriette although it would be an easy mistake for Harriette. Names were often less firm than they are nowadays!
Hope this helps a little
Andy