Hmmm. That's an odd one. I have never had any angelfish so I will promote your post so hopefully it will catch the attention of some angelfish keepers.
If you don't get a response quick enough or the status of your brother's fish change, just update me and I'll see what I can find for you. I am pretty good with diseases, but not so much with odd things like this. Sorry.
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Phishie

"An ounce of prevention is worth a pound of cure."