One of bad energy surrounding them. Preferably not black magic. And they're unwilling to partake in the spell/ritual (negative energy doesn't want to leave). Id wager it's not a demon/possession. Just a negative energy surrounding them for a long period of time. Any ideas?
I would first start out by doing some divination to dig deeper. Maybe try to find the root of.the problem. If its a recurring issue in his life than maybe cleansing is not gonnna resolve his whole issue. I am unsure how he got the negative energy so i am just pulling out ideas. He could do a spiritual bath for a certain amount of days, but how many will.be decided upon the reading you will do. I would recommend taking a bath with cleansing herbs like hyssop or burdock root. Boil the herbs to make an infusion and then let it cool. But if you have no herbs just try salt and water. Recite psalm 23 over the herbal water or salt and then add it to the water. I hope this helps! <3