It was stupid to not tell him (litterally out of character, will be interesting to see how this plays out next episode, because Jon and her acted like it was normal afterwards). But what he did in that battle was even more stupid. Its one thing to go after your brother, but what he did afterwards was MORONIC. It litterally lead them to the slaughter. Your brother is dead, go back to your army and stick to whatever unprepared tactic you have, not rush 6 thousand men on your friggin own, forcing your half strenghth force to rush into death trap with you.
And she wasnt giving him solution, and was bit abbrasive, but she was RIGHT, she told him how are you making battle plans according to an enemy you dont know? He was litterally understimating Ramsay as some headstrong fool who is governed by emotions and pride alone, kinda projecting...She was the only one in the room who knew Ramsay well, but they or mainly Jon was talking like he knew him exactly after 4-5 minutes of shitty bravado talk. She admitted her shortcomings of not having battle plans, but told him Ramsay is the opposite of what he thought he was, was being played and she was telling him to be careful. As we saw, he litterally fell for it all.
I left this episode being ALOT LESS impressed with Jon then I was before, even if he conducted himself well as a warrior, but as a leader he was completely idiotic.
I know Sansa is easier to dislike, but come on, the bigger fool on this episode alone was obvious.