Oh wow, so many directions you could go. What's the riders color scheme? Could get something with an accent in that. Do they live in a warm climate? Maybe get something with cooling air flow sections. I'm sure people have favorite brands and whatnot but I always bought based on fluffiness and color.
This was the fluffiest pad I used for shows:
https://www.amazon.com/dp/B003MYCZH2/ref=cm_sw_r_awdo_navT_g_4CYK91Z1HG7EX774ZW54?psc=1
I even gave my extra one to a friend after my horse retired and she LOVES the pad too.