There is no point of no return, but obviously the further into the transformation they get, the more power it'll take to heal them. Also, keep in mind that no matter how human-looking they're returned to being, the healers can't remove the wendigo spirits from them - the only things that can do that are death and Hope. Healing the body can revert the character back to the initial stages of the transformation, into a human form and a mindset that is more human, but they will still feel anger and hunger until the spirit is removed.