I don't know too much about diseases and I do not QT like most people think of QT. However, I have had lots of Watanabei angelfish. Most issues stem from too quick of rise dealing with all kinds of stress from swim bladder damage and subsequent infection.
In general, these are hardy fish that stand as good of a chance as any if you can get them to eat. Mysis, capelin roe and other fine meaty chunks are good. These are planktivores, so you have to have enough flow to keep the food in the water column, but not wear out the fish - so a bit more flow during feeding can be good. They have never wanted to pick around the rocks for food, at least when new - picking is an acquired skill for them. All of mine quickly took to NLS pellets if 1). they were soaked and soft and 2), some other fish in the tank showed them that it was food. I do not QT in a traditional sense and each of my introduction tanks has live rock, sand and a few really nice fish that help the new ones come to the top when they see me, eat pellets and all of that. I have a smaller female now and it loves to hang out with a purple tang and orange lined rabbitfish that are kinda my welcome wagon until they get larger. They seem to like tank mates, at least at my house. I have this one in a 40 breeder at about 3 inches and it is totally fine.
With the larger "chin" and size, that looks like a terminal male to me. However, if you lose the female, I would get another female Genicanthus in there ASAP... any kind usually works. I had a Watanabei female change to a male with a female bellus, among other types... including a female bellus change (BOO!) with a few swallowtail females. If it is indeed terminal, then you are good without another.
In general, these are hardy fish that stand as good of a chance as any if you can get them to eat. Mysis, capelin roe and other fine meaty chunks are good. These are planktivores, so you have to have enough flow to keep the food in the water column, but not wear out the fish - so a bit more flow during feeding can be good. They have never wanted to pick around the rocks for food, at least when new - picking is an acquired skill for them. All of mine quickly took to NLS pellets if 1). they were soaked and soft and 2), some other fish in the tank showed them that it was food. I do not QT in a traditional sense and each of my introduction tanks has live rock, sand and a few really nice fish that help the new ones come to the top when they see me, eat pellets and all of that. I have a smaller female now and it loves to hang out with a purple tang and orange lined rabbitfish that are kinda my welcome wagon until they get larger. They seem to like tank mates, at least at my house. I have this one in a 40 breeder at about 3 inches and it is totally fine.
With the larger "chin" and size, that looks like a terminal male to me. However, if you lose the female, I would get another female Genicanthus in there ASAP... any kind usually works. I had a Watanabei female change to a male with a female bellus, among other types... including a female bellus change (BOO!) with a few swallowtail females. If it is indeed terminal, then you are good without another.