Here is a tutorial for client side mods. Warning this involves changing that actual Java (.jar) library. So might want to make backup of it first.
http://gurpy.com/forum/index.php?threads/guide-installing-client-side-mods.139/Please note these client mods are meant to be used on your client/local server. They may not be allowed to be used on remote servers. Usually what happens is the client will crash/drop connection when using a client mod if the server does not support it. Check the mod to see if it can. Some also require a server mod as well.
He may be wanting to simply add a texture pack... those are much easier to do as you can simply drop the texture pack zip file into the texture packs folder. Which is located under %appdata%\.minecraft\texturepacks
Also want to make sure he is not mixing up a server mod. As those can only be run on a server. (specifically a bukkit server). If he is interested in a server mod, let me know and I will see about getting it installed on my server he plays on.
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