I don't think it even needs to be that closely related to the course, Molly. A year out there in the real world is beneficial in so many other ways. It can focus the mind, showing you much more clearly what you want to be doing after uni. That can make you think again about the course you're planning to do and whether you want to change your mind. You will also learn so much more about responsibility and independence, and you will head off to university with, hopefully, a much more mature attitude to life and study.
Perhaps your friend can find some animal-related work or voluntary service for a year, not necessarily with a vet. He may find that rather than becoming a vet, there's an animal route he'd never thought of before and which perhaps appeals more.
It's also a year's life experience that other candidates won't have!