Broken knee cap

My 87 year old mother broke her knee cap over two months ago.
She has undergone two operations as the knee cap did not hold together after the first one so the orthopedic added more wires to hold the knee cap together.
She was put in a leg cast the second time.
Two months later the full leg cast has finally come off. She just started with a physical therapist coming to her house.
Her knee is still very stiff and she has not completely bent it.
The therapist wants her to lie on her stomach and he will force the knee to bend.
He said this is the only way to get it moving again.
I would think gradual exercise would be a lot less painful. Can anyone please advise me? Thank you!