It's definitely a good idea to check the specs for the class in the prize list for the show. The class may be equitation or a hunter class. The advice you've been given about finding the center of the "jump", staying straight, using corners, etc. is all good and will benefit your daughter in both the Eq and Hunters. I'd be confused too if someone was talking about diagonals at a walk! Maybe there was a diagonal line in the course? Rather than just staying along the rail, your daughter rode across the diagonal of the arena?
Without seeing your daughter, it hard to say wxctly what could be done to improve placings, but focus on correct position (heels down, straight line through shoulders, hips and heels, straight line from elbow to bit, etc) as well as demonstrating adequate control on the pony (turns when asked, doesn't sulk by the gate, etc) are all good things to focus on. In the hunters, the pony may be hindered by not being the fanciest type, but being very consistent, with the same pace, pleasant expression, willing an responsive to rider, will go a long way in lower level shows.
Posted via Mobile Device