I also think that cutting her off would be the wrong message about how we should deal with love and loss. If you found out someone you loved was terminally ill, would you stop seeing them to try to reduce your own grief? Is it better to not ever have a beloved pet, because you know they will die eventually?
It is much better for your son to experience losing her because of life circumstances, than because of an action you took.
You use the phrase "nip it in the bud," but that implies something you could stop before it has a chance to get started. But (to switch metaphors), that horse has already left the barn.
