Dear person writing to your boss. It doesn’t matter if Fergus makes you upset. You’ve given notice. Let go.
Fergus didn’t just walk in the door - somebody liked him and his background enough to want to hire him. Your actions are undermining that higher-up’s decision. His supervisor may put her hand up to stop his input, but it’s inappropriate for you to do so because he is your equal. It is also inappropriate to speak harshly to him in a meeting setting. That’s what closed doors are for. What you’re really doing is trying to make yourself look more valuable going out the door and that’s selfish. Turn your criticism into a helping hand.
Fergus needs to let trainees know that he is the new trainer - because he is. You’re gone. If he just sits there while you decide whether he’s your perfect duplicate, you’re wasting the company’s resources. He should be conducting the training sessions, with you there to give him feedback - after the session in a quiet environment. Since he’s done equivalent work in other settings, and likely has a wealth of experience to draw from because of his age (btw, even mentioning his age in a letter to your boss is harassment), understand that he may have different insight than you. His responsibility is to please the hiring authority, not you. Have fun in your new career and don’t worry about Fergus. He’ll be just fine.
Great answer to her letter. Spot on.
Wrong.
You need to go back and reread the article. SHE is the trainer. It is her NEW position. Fergus is the trainee, hired to fill her OLD position. SHE has been tasked with training HIM.