First of all, I'm sorry your niece has to go through this. It is a horrible experience, regardless of anything else. It hurts like heck and it takes time to get over the shock.
But if she were my daughter, let's say, I would think that natal Saturn - natal Mars played its hand in this. Your niece has natal Mars opposite natal Saturn, so there may be a tendency in her to have a very strong and harsh personality at times, maybe even domineering, aggressive, or even bossy.
When no reason is given as to firing, as related to work performance, I think it usually comes down to personality. It could be that somehow or another she rubbed someone the wrong way - and that someone decided they did not want to work with her over the long haul. So her work performance was as you described, but there was personality conflict. Now, who is going to come out and admit that? That rarely happens, in my experience. I bet your niece might not even see how she could come across to others as I described her tendency.
So, what else? Her natal Mars and natal Jupiter share the same sign (planets within 6.5 degrees of the beginning or end of any sign are considered to be in both signs - here, that sign is Virgo). Transiting Mars and transiting Jupiter are square - and because they share the same sign, these planets are activated - and negative. It is transit Mars in Aquarius and natal Saturn in Aquarius-Pisces that made these two planets active. And next you see that transit Saturn is square her natal Jupiter.
So Mars-Saturn is active and so is Mars-Jupiter - and the Saturn-Jupiter is activated because of the prior 2 activations which link them. Is it possible that your niece, under these negative influences, allowed the negative tendencies of her natal Mars-Saturn to come out and perhaps found fault with the other's past work and maybe pointed it out or changed something to make your niece look better than her colleague, thus perhaps ticking someone off?
In addition to the above, transit Sun was square to natal Jupiter and opposite transit Saturn, adding more negative energy to the situation. The Sun represents those in authority. I wonder who in authority she might have inadvertently rubbed the wrong way? Also, transit Moon was square natal Saturn and transit Moon and transit Saturn share the same sign (Sagittarius), thus these planets are also activated.
Jupiter “negative” Saturn governs faulty thinking and/or troubled judgment and just bad timing all the way around because of gloom, sadness, or loss of confidence; a real downer.
There is no doubt she worked hard (natal Sun trine natal Saturn). But sometimes that just isn't enough when you rub someone the wrong way or make an error in judgment.
I wish her all the best of luck in the future.