I understand your concern. In situations like this, "for better or worse" sounds quite challenging and it is a personal choice whether you want to take this challenge or not. However, from a doctor's point of view, helping someone through bad health and addiction to a better quality of life can be quite satisfying and emotionally rewarding. I can only imagine that this satisfaction can be greater when the person you are taking care of is someone you care about on a deeper level.
That said, yes, it is clear you fiance has an addiction to Percocet. Anyone who takes this drug containing opioids, for more than two weeks can develop a physical addiction to this drug.
The first step towards treatment, is to get him to accept that he has a problem. Treatment involves either opiate
replacement therapy, with methadone
or, alternatively, a short stay in rehab for a detox followed by longer treatment.
I hope this helps.
Wishing you both good health and all the best.