Since you can never be sure if the new plants you bought from the store or given to you by a friend is not infested by parasites, it would be more prudent on your part to just sterilize the plants before exposing your koi to them.  

Sterilizing plants is not as difficult as many people think.  You just need a container to hold the sterilizing solution in.  The holding tank that you use for your sick koi is a good choice for this purpose, assuming that there's no koi recuperating in it of course.  At any rate, any clean container will do.

Put 20 gallons of water in your clean container.  Add 5 drops of formalin to the water.  This will be your sterilizing solution.  Immerse your new plants in this solution for 5 to 6 hours.  This should kill the parasites, snails, and fish eggs that might be attached to the plants when you acquired them.

Make sure that the plants are totally immersed in this solution.  Parts of the plants that are sticking out of the water will not be sterilized, so the purpose of the whole exercise will be defeated if this happens.

After this immersion process, take the plants out of the formalin bath and rinse them very thoroughly in running water for several minutes.  Remember that formalin is very poisonous to fishes, so there must be no traces of it when you finally add your new plants to your koi pond.