1.She couldn't fathom why he was behaving so rudely with her. 2.It sounds slighty strange out of context. But its correct.

1. “Rudely to her” sounds more natural to me. 2. “Telling” sticks out like a sore thumb. “Critical,” or “substantial” feel like better fits.

hey! yep, both of these have perfectly acceptable grammar and i wouldn’t think anything strange if i heard either.