Despite being a master armorer, Yoana became Fergus Graem's assistant because many would never believe she, a human female, would be capable of making master armor. Fergus, being a dwarf, was far more likely to be perceived as a master armorer and thus the two struck an agreement for her to be his assistant, but she was the one crafting all the complex pieces.
When the local Nilfgaardian commander came by and ordered a suit of armor that could withstand a shot from a crossbow, Fergus stated it couldn't be done and Yoana, having enough, stated it could actually, with the right tools.
If Geralt helps Yoana out and wears the armor: With the special tools from Skellige and help from Geralt, Yoana created a piece of armor that, true to her word, effectively repelled crossbow bolts while Fergus' didn't, showing everyone who was the real master armorer. Impressed, the commander promoted her to be the head armorer and Fergus, saddened, got ready to leave. However, she offered for him to stay, but this time in the appropriate roles: she as the master armorer and Fergus as the apprentice, and the dwarf agreed.
If Geralt refuses to wear the test armor: Having nobody to test her armor for her, the commander pronounced her as a fraud. Upset, she packed up her things and left Velen for parts unknown.