Dear xiaobaitu, please answer this question

my friend is going to get her Ph.D. later this year, she wants to apply for green card after she gets her degree. Everything seems fine, but her boyfriend is planning to go to China to work or study for 1-2 years. She wants to go with him, so she readily come up with a plan of action here:
1. Apply for green card first thing after graduation (I assume she means NIW 140);
2. Hopefully she is lucky enough to get approved very soon;
3. Go back to China for 1-2 years;
4. Return to US with a green card ready or nearly ready (no time wasted in waiting here in US);

I told her, Sounds good, however, too good to be true, but because of my limited knowledge, I can not point out the problem here.

please help me out, Xiaobaitu
thanks a lot