Oooh mama I had a great idea for my next post. Gonna post full details here first though beforehand since it's that kind of thing.
In short Ember is going to explain why they should let Nemesis complete her objective WITHOUT a fight.
EDIT: Ok here's the logic...
So I just saw a way that an AI can work which would make alot of sense here.
http://www.engadget.com/2015/06/17/super-mario-world-self-learning-ai/Can't believe I didn't comprehend this before but it can help out now.
So Nemesis probably has a similar learning system; She goes through and analyzes what works and doesn't work through trial and error as well as analyzing statistical odds. So with that in mind... Nemesis right now is actually really "stupid." As in she doesn't have much to go off of and therefore is really inefficient. Ember, being who she is, would notice that Nemesis is not at a high generation for combat processes, therefore making her not very effective.
Now with every combat experience, she would rapidly evolve that part of her neurological systems. So the more she fights, the better she will obviously get. However Ember knows how fast Nemesis will evolve and therefore knows that if they fight Nemesis now, they will be giving Ragnarok a powerful weapon since knowledge is power and Nemesis will learn how they fight and improve upon it rapidly.
So tl;dr Ember would warn them that they shouldn't fight Nemesis unless they wanted to give Ragnarok an efficient weapon to use against them. If they let her go then Ragnarok might think that they have such a weapon when they actually don't therefore making them overconfident plus having someone on the inside on their terms will give them the ability to destroy Ragnarok from the inside out.