1) Stopping enemies - Makes it easier to jump or kick these enemies without ANY fear of getting hurt. So if you're down to 1 health (3 max is the norm, 6 with powerup) then you don't have to worry about getting angled for that jump that gives you a coin.
2) Stopping some projectiles from enemies - any level where you have to reflect projectiles, coconuts, etc, at an enemy, normally you have to do this WHILE dodging other stuff and this ensures it won't pass you by while dodging something.
3) Collecting Starbits - It's just generally nice to not have to worry about collecting the damn things, one less thing to worry about and you'll be almost absolutely certain to have enough starbits to feed ANY of the star kids along the way. This is even more apparant on the controller minigames like with the manta ray levels or the ball rolling levels.
4) Freeze shrinking platforms - this becomes MUCH easier to navigate these things when you don't have to worry about them falling out from under you.
5) Make mario jump - every so often, you might have not quite made the jump at the right angle, if she's quick at it, there's a good chance she can give you that extra jump you need to get back on the ledge and not fall to your doom.
6) Didn't even know about being able to stop the fireballs from bowser, that would have been a huge help in kicking his ass.

There will never be a point at which you NEED the 2nd player, but I can't think of more than half a dozen levels that I didn't benefit from having a second player.

Oh, and provided you can refrain from being a douchebag, then you should be able to enjoy helping her play through as well and pointing out things she might miss, actually you'd probably enjoy it even if you were a douchebag, but if you refrain from doing that, she'll be able to enjoy it too.

Hmmm, I didn't know it could do all of those additional actions. I just found it annoying and in my way. If I knew that they could help out so much with the enemies I would have allowed my brothers to play. :)

I love messing up my brother when he plays, making him jump and wasting his star bits.

Two players is awesome, especially if the second player isn't too good at video games. There are a lot of times when you'll be glad that second player is there to save your ass. Give her the second Wii-Remote. She'll figure out what to do in no time. Just make sure you stress to her not to waste all of your Star-Bits.

Wow, I didn't know there was so much you could do with the 2nd player controller. My gf likes to use the 2nd player controller while I play, but all we thought you could do was just collect star bits. Maybe we should have looked at the manual closer(at all)! ;-p