The idea of the whole game is "Get the other person to fall off the platform" Why would you let him come back?
Always this is my line in a topic like this. Looks like someone got there before me.
Invincibility Complaints:
Let's face it, though, invinicibility. That's something to use to your advantage. It's not like you HAVE to get hit, you can dodge attacks and evade those few invincibility frames. Everyone does it. Whining about it is going to get you killed if you want to play seriously. Running away? Tactic. In free for alls, that's what you do. You run, steal a kill at high percentage, and evade attacks, single people out, KO them. At the end, you won't be hurt much, kill the last guy-- you are the winner.
Edge:
There is no such thing as FAIR play. This game is punishing someone for a mistake they make. So, your technique might get more repetitive, but it is effective. Your friend is a bad loser, and is too nice to be playing a fighting game. I mean, come on, he can't be a good player if he doesn't do anything effective. You can't play fair, that's being too nice. You beat your enemy-you will do whatever it takes. If he's got a problem with edge techniques, why is he playing the game? You send the opponent off the edge. It's just humorous to let them just get back on every time. He probably will have a hard time VS a Samus if he doesn't even defend the stage, defeating the purpose of the game...
If that doesn't convince you, look at pro videos. They do it, it isn't banned. Why ban it yourself? Things get banned for a reason, if the reason goes beyond your understanding, just trust the other player's judgement. It's fun, and it takes skill to not SD by the edge and make effective guarding skills. Lots of cool comebacks are by the edge. Try it out, anyone can do it, so it isn't unfair.