Hi...This hasn't happened to me (or my fish, rather!) but I read in the May issue of Aquarium Fish magazine that common causes for cloudy eyes are cataracts (which does look like your pic), internal parasites, bacterial infection, vitamin A deficiency, or low pH.
The article suggests to test the pH, since low pH is the most common cause. If it's lower than 6.8, and especially 6.4, do water changes to bring the pH up. Does the tank have a lot of fish, because a large bio-load can cause pH to drop as more fish wastes create more acid in the tank to cause the pH to drop. Water changes should clear the problem up.
If water changes don't help, try an antibiotic treatment for a bacterial infection. Make sure the fish is getting a good diet. There are vitamin supplements you can add to the water. I've seen liquid vitamins on the DrFosterSmith.com site, but I don't have any experience with them myself.
I don't know what to do about a cataract! I hope your brother-in-law's fish gets better. The fact that the fish is eating normally and has no other behavioral changes is a good sign.