My friend has come to US on R1 Visa.He is currently working as priest in Hindu Temple.He got Green Card before 1 year through the current organization. His family has just got the green card. He is staying in US from last 5 Years.
Question:
1. He wants to change the employer.i.e Move for work at different temple.He told the same to his current employer. His employer is saying that he will cancel the green card if he leaves.
Can his employer really do that?
Can he move to different employer? Is there anything he need to worry about or take care?

2.He signed a document which says he need to give 60 days notice period before resigning. What actions can the employer take if he leves immediately.

I am asking this question on behalf of my friend as he is worried.