Yes, he did take roles away from Mejia, which is why Mejia told Farrell he was going to quit - he'd lost yet another role. Which is still not the same as saying that Balanchine fired them. Making life uncomfortable for an employee is indeed one way of showing him where the door is, but Farrell was still dancing all her roles and receiving the occasional compliment from Balanchine, although he did have her begin teaching understudies, which she had not had to do before. If Mejia had gritted his teeth and stuck it out, who knows -- but given that he was not made welcome when Farrell returned to the company, we can make an educated guess.