What attire for horse presentation: English or Western?

I would go with English simply because whoever decided that you needed to wear a vest and shirt for western has obviously not seen any current western style/show trends. Wearing a vest would make you look outdated--not the look you want to have while giving a presentation and trying to appear knowledgeable. Sounds like you're closer to having an English outfit, anyway--all you lack is a pair of breeches for your friend. You can get a cheap pair for $20-30 in most catalogs or I'm sure she can borrow some from somebody for just one competition--especially since she's not riding in them and therefore won't be getting them dirty or ruining them.