No, this SP they borrow from you shows up in the “delegated” SP in their wallet: it’s in parentheses, not in their actual balance. For example, the number in the parentheses of most people is a positive number because they got delegated a small amount of SP by @steemit when they join; this small amount of SP is the bare minimum amount you need to do basic things on the site, the minimum bandwidth if you will. The number with somebody who is DOING the delegating is typically a negative number, like the one in the parentheses in my SP wallet. They could power down, but it would only power away the number outside the parentheses. Really good question though man 😄 I feel like that would be a gigantic case of theft immediately lol @MinnowBooster would get so much flack if that happened! 😂