Wife trains her successor

I feel like this story must already exist, but unsure of how to find my way to it. If it does exist in some form, please let me know where it is and ignore the idea below haha.

A wife is diagnosed with a terminal illness. She and her husband have had an open and understanding marriage blessed with riches, children, world travels...and great sex, including swinging. The wife is resolute in finding and training a young women to fulfill her husbands needs; not as a second marriage but a fun partner for the rest of his retired life. Husband and wife enjoy the last of her healthy days revisiting their young romance and wild sexcapades during the "training process".

I like this idea,though dealing with the sadness aspect would be tricky.

Yeah, it's weird at first glance, but under right circumstances make shocking lot of sense. Sadness? Sure it's there, but this is story about acceptance, I believe, including but not limited to, our mortality.