There are several places called Trethewy/vy in Cornwall (there was probably very little difference in the pronunciation at one time. The surname Trethew(e)y is taken from the one in Lanlivery, map ref. 068570.
Robert Trethewy - one time warden of Exeter College - was of the family that lived here and any association with Ruan Lanihorne is unjustified speculation. You will find an account of him Newsletter No. 11 of the Trethewey Society which should be available at any of three places in Truro: the County Record Office, the Courtney Library and The Cornwall Family History Society Library, 5 Victoria Square. Alan Kent, organiser,Trethewy Society