It's been a while since I've read the books, but are we talking about the incident where Egwene summoned Dreamrapists to attack Nynaeve because she almost inadvertently unravelled the web of lies and deceit Egwene was profiting from in front of the Wise Ones?
Sort of. I believe the one where Elayne and Nynaeve both were in danger of revealing Egwene's deceit in respect of the Wise Ones was a different time, where Egwene more of less rode roughshod over them in the conversation subsequently, and left them.
This instance, which did include the nightmare, was where Nynaeve was asking questions when only Egwene was there, but were potentially going to open the same can of worms as above. Egwene took the opportunity to try and show Nynaeve just how dangerous the world of dreams was, as Nynaeve at that stage was quite cavalier (and continued to be so really until her last confrontation with Moghedian).
The tea came about as Nynaeve was trying to skirt around a lie she told, and Egwene then used a treatment that Nynaeve used to use on people when they were lying, to force Nynaeve to tell the truth.
Following that, there are many points in the following two books at least where Nynaeve goes to lie, or at least skirt the truth, but then has the taste of tea in her mouth and subsequently tells the truth instead.
Egwene's methods weren't good, and her motives less so, but I think Nynaeve's subsequent behaviour did change due to it, whereas there was no one to do it for Egwene.
Outside of her time with the Seanchan and the Black Ajah in Tear, Egwene only received punishment / ill treatment when she chose to do so, like with the Wise Ones or the tower Aes Sedai, whereas Rand, Nynaeve etc received it willing or not.
I just feel it isn't entirely fair to judge Egwene too harshly on this, when if she went through what the others went through, or had people forcing behaviour like she did for others, she may have changed her ways too.
