I rode in college in IHSA - is that what this school has? She'll have to change up her position a bit, that's all. With her riding background, she'll probably get placed in the walk-trot-canter division. We had a girl who showed American Saddlebreds to a very high level and then did hunters with us in college, and she adjusted just fine.
I don't know much about saddleseat, but to me it looks like they ride with their hands higher (which makes sense since the horses carry their heads higher) and with their leg sort of out in front of them, so she'll have to lower her hands, bring her leg back under her, and in hunters we ride in a lighter seat (more on our crotch instead of on our seat bones, with a more forward hip angle). The only other change I recall our saddleseat rider having to make was that she put on a ton of makeup on her face at her first show. Seems like in saddleseat and western pleasure, they like the makeup and such? But in hunters, we don't need makeup.
Wanted to add, in hunter equitation classes there aren't any patterns usually. Just rail work. Occasionally a judge might test the riders with a short pattern for regionals, zones, nationals, etc, to help place the top riders, but typically not.