If I lower the difficulty all the way down enemies takes way fewer hit to die, does that make me lvl up slower? I would like to lvl up slower somehow because my girlfriend like to make potions and collect ingredients, but if she fights too much she will lvl my char up too fast.

Does lowering the difficulty work since she kills things faster or is it the kill itself which gives xp and not every slash with the sword/hit with arrows?

Why not just create a new character for her to use and swap the save file folder each time before playing your character or get a save manager mod to keep them separated. That way your character is not effected by what she does !

I don't know about Skyrim, but I know that with other games like Oblivion and Fallout that when you lowered the difficulty, you didn't get as much xp for doing things like killing stuff.

yep. takes longer.

Well, I kinda want the restore potions she makes for me. But good to hear it takes longer.

Turning down the difficulty makes leveling a lot slower. When I started a new character on novice difficulty by the time I got to bleak falls Barrow I was only lvl 3. However when starting a character on expert I got to level 5.

Yes, I assume is because in higher difficulty a fight last much longuer, then you rise your attributes a lot more. If in a normal fight you have to block once time, in an expert one you have to block 3 or 4 times. So, you will gain a lot more experience from that.

It's every hit. The XP you gain from hits are difficulty independent, it takes longer as novice only because of the less hits required.