I suggest the wife help her nephew to apply for colleges this fall. After he got admitted, then let him apply for visa.
99.9% of chance, he will be turned down. The end of the story. Let them know ahead of time it is hard to get visa, he should plan to attend a college in China in the same time.
I do see young kids from China who got visa to study in college/even high school here, but the majority (99%) has a wealthy family(high officials or CEO of a big company) in China.
I helped my young brother, my young sister, to study at college here, they both were turned down for visa. They eventually come here for graduate school with fellowship.
You can only help them so much. The nephew has to learn to help himself.