I am doing the quest in High Gate Ruin and the other mage used a fireball that hurt Lydia and now they are in a battle to the death, any way to stop them before they kill each other.

reload... sorry - but sheath your weapon and if you have a spell that will work, calm them - can't recall the higher level one - is it Pacify? I don't know if that'll work. I had to reload that one. had the same thing happen - i was using Aela.

Don't bring her with you and you won't have to worry about her attacking. Followers get in the way more than anything. It's easier to fight alone.

You could try a calm spell on one of them and order the other to return home, then just go back and get them.

Open the console and click on Lydia. Type the following:

stopcombat
sifh 1

PC only, of course.

That quest (which I just did last night) is annoying. For those who don't have the console, you have two good options:

1. Don't bring your follower(s).
2. Don't bring Anska.

The second works because there are two ways to start the quest: talk to Anska, or pick up the scroll she's looking for. In fact, the second is better, because Anska is one of the most annoying temp-followers in the game:

-She detects your stealth. Barbas also does this, and it's really annoying if you like stealth.
-She is as pushy as Barbas, and will try to occupy your position whenever you stop.
-If you are in detected-stealth and move any direction except directly away from her, she gives you the pickpocket warning.
-She panics every time you use arrows or magic while she's glommed onto you.
-If you have followers (even temp followers like an atronach,) she's more likely to use Fireball than Firebolt, despite having both. This angers your real followers.
-Her combat AI is, believe it or not, melee-oriented: she's quite happy to charge in while casting Fireball.

So just start the quest by getting the scroll.

FUS RO DAH!!

Bias refereing.