He's trying to learn, but I have no idea how to teach him well.
This is my mario...
http://www.youtube.com/watch?v=7PAAUymcrsM
This is his mario. What exactly should I have him focus on?
http://www.youtube.com/watch?v=LCqzB3_xQSA
First off, I apologize a lot as well whenever I hit my team mate or mess up.
First, you can tell him to use the f-tilt more while his opponent is next to the edge because it's safe 95% of the time and if they block it, you won't get punished.
Secondly, tell him to practice his tech skill. For instance, I watched his Mario wavelanding. It's good, but if he can get the full distance off, then he can learn how much distance he needs at the right time. Same applies with a wavedash.
Third, it's good to throw the opponent right away unless you really have to think cuz that'll give them time to think also. If you throw them right when you grab them, they'll most likely DI a way you would want them too, or better yet, not at all.
Fourth, he needs more speed, and tell him d-smash is the fastest smash and usually can't be punished if the opponent shields.
Other than those 4 things, tell him to keep it up and to make a smashboards account =P
And if he already does have one, tell him to post anywhere on the Mario boards for help.