Well, he was going to leave, but now that she's told him to, he won't. Spite is a human emotion that even the great Professor Oak is capable of sometimes! Her ego doesn't need more stroking, anyway. If he were more spiteful he might be tempted to pick another fruit just to annoy her... but no, he respects nature too much to do that.

"You know, despite your bad attitude, I was still perfectly willing to cooperate with you when this conversation started. Maybe a little self-examination is in order, to determine what you said or did to change my mind." It didn't have to turn out like this; one personal attack too many was the straw that broke the Camerupt's back, and even a peaceful, gentle man like him has his limits.

With that final opportunity for education sorted, he takes a few steps away, just out of conversation distance... and sits down on the ground as Pidgey flies back to him for comfort. He pats it and gives it gentle scritches, making soothing sounds.