Anyone who has US permanent residency (a green card) can do anything she wants to do now. She can work at research, open a coffee shop, train dogs, drive a school bus, or marry a rich guy, raise their children and perform charitable acts. She has every right, freedom, protection and obligation that a citizen has, except that she can’t vote in most elections and can’t run for president.
So “Is it hard to get US citizenship after the green card process?”
No. Far from it.
Your friend can apply for citizenship, if she wants to, when she has had permanent resident status for five years. The process involves some paperwork and a lot of waiting around, but it is automatic as long as she hasn’t commit any crimes that might make her ineligible. And changing her profession is not a criminal act.