Can one enter usa on b1/b2 visa while green card is on file?? Is it possible to change status b1/b2 to green card??

If she has a pending consular case for permanent residency, then she cannot enter on a visitor visa. If she is entering on a visitor visa for the sole reason of adjusting status after the fact, that would be against immigration laws.

No, because her category is oversubscribed and there is a backlog of a little under 2 years. The purpose of the B-1/B-2 is not to serve as a bridge to permanent residence, but to allow temporary visits. Retain an attorney to make sure she avoids any suspicions of visa fraud.

You should speak with an immigration attorney to avoid a potential misrepresentation charge.