You'll need to find a school that will be able to issue I-20 for foreign students to apply for F1 visa, so you'll need to find a private school that will be able to do that. Most private schools don't do that, especially for elementary schools.
I don't really understand why your friend is doing that!
I wouldn't recommend it until at least 13 to 14 years old if the kid is mature enough to take care of himself/herself!
One of my friends is the guardian of her 13-year-old niece from HK. Although her niece is living with her parents (i.e. her niece grandparents), she felt quite some responsibilities and a little pressure as her role is equivalent as her niece's parent in here. But that's her own sister's daughter.
So you might wanna think twice!
If s/he's going to a boarding school, it might be a little easier for you as a guardian.