If the pond has a UV light, they should change the bulb every year. That will help the agae. If it doesn't have a UV light, they should get one.

They should also check the salt level. There's a cheap salt test kit that can tell them the level. Even though they are fresh water fish, Koi need salt to fight infection.

When adding fresh water, make sure to add a De-Chorinator (sp) You can get it at Home Depot or Lowes. Most fresh water from a hose has chlorine in it and it can kill fish.

Tell them not to over feed the fish. They eat algae and bugs despite them always acting hungry.

Don't feed them when the water temp is below 55 degrees.

I'm sure there's a ton more.