I haven't read all of the responses, so hopefully I am not repeating anyone or behind the time. But there are two thoughts I have, and it depends on the people involved
1 - slow breakup... cauterize the wound before it begins to hurt to much. This may seem harsh but ending it instead of having a long drawn out breakup will begin the healing at a potentially better stage.
2 - The belief that all relationships are fluid - This reads well to me, and in fact I have done in practice, but it can be a painful process. Allow the relationship to go to where it goes, but understand that all relationships are fluid. To hold onto a specific relationship a certain way can cause heartbreak. I see a lot of this in long term poly's. Something of a zen moment I guess.
Neither is great, but both allow the person who is suffering to regain control.