How does EXP work here

So I'm doing this map areas with 2 other friends. I'm lvl 76 and my two other friends are 53 and 74. The guy that is level 53 gains no EXP whatsoever. I thought I can use these maps to help him gain quick exp

There's some maths involved, but in basic terms, when you're partying you get a higher percentage of the total xp the higher your level compared to everyone else's, and it scales pretty sharply. The intention is to prevent exactly what you're trying to do.

If you want to read up on the maths, see here.

On top of that, there is an experience penalty for being too far above or below the level of a zone, which also scales very sharply. Again, the same page I linked above has details of the maths.

That's exactly what the game doesn't allow, if there's a too large gap between player levels the player with low level will get an experience penalty. You can see the exact formula on wiki if you're interested, 'experience' page.

so if he clears it himself, it's fine?

Due to him being so underlevelled for the zone and the regular experience penalty thus kicking in, it would still be more efficient for him to clear Ledge or similar.

I think if you are in the vicinity it will split the experience (giving you about all of it) even if he does the killing. So you can't buff him with your auras.