My friend's 12 year old child is constantly getting low grades in school. Any tips on how to persuade a child to study?

You may suggest your friend to make studying at home fun. Don't ever make the kid feel that reviewing is another burden to surpass, aside from school. As much as possible, studying shouldn't feel like an added workload. It pays to help the kid so that he/she feel a little support and don't forget to encourage. Should he/she need some alone time in studying, that's a good sign and take advantage of it.