What you have related sounds potentially fine. However, you are addressing a very superficial aspect of overeating and that might be why therapy does not seem relevant to the client. Is it really the case that this person doesn't know when he/she has had enough???? Pretty unlikely, really.
So therapy might better be addressing matters of deeper relevance such as illustrated by his or her psychosocial developmental concerns: loneliness, denial of feelings, rejection, substitution of food for love, etc.
Lacking such relevance, hypnosis aimed at a simple reversal of symptoms is shallow and will yield responses that it is not feeling relevant to growth.