For gushing about some guy which clearly made him uncomfortable? Especially if she's gonna continue with this "we're in a pretend relationship" thing, telling the guy who is more than a friend, less than a lover, how hot and cool your mangaka is, is pretty shitty.
You said it right, they are
pretending. They are just friends, the whole act is just "to become a better mangaka" without ulterior motives. At what point it became a trial for a serious relationship? Never.
And he was uncomfortable because he started to have feelings for her, but how could she know? 🤷🏻♂️
Could she have started the conversation talking about work? Maybe.
Could she have still talked about sensei in a second moment? Also maybe.
But what difference could have it made? Absolutely nothing because he would still feel uncomfortable and again because of his newborn feelings that she can't know about