20% in a year seems like a ton, but if they're 60% bodyfat right now, it'd be more reasonable than if they were, say 25% bodyfat right now.
for continual long termfat loss, and not losing any muscle mass in the process, I doubt you can do much better, on average, than 1lb of pure fat loss per week. at the beginning it'll go fast, but at the end it'll be a crawl, and losing lean mass will be a little harder to fight off
*edited to clarify*