Can A Son Get His Share In His Mother's Flat?

Hi I have a boyfriend he stays with his mother in a flat which is completely on his mother's name. He has an elder brother who lives separately with his wife. His father died 3 years ago. His father's family gave 10 lakhs of his father's share to these people to take this flat in which currently they are residing. So the money from which the flat has been taken is of his father's. My boyfriend's mother soon after her husband's death showed her true colours. She started having affairs with other men. Currently she is having affair with 3 men . We have full proofs like whatsapp chats and her call recordings regarding her affairs. Her sons know about this affair and thus they don't want to keep any relation with her and want her to sell their flat which is on her name and give them their share. Their mother couldn't be loyal to their dad and so they are taking this step. But that lady is not giving them their right, she said she will not give them any single rupee. We want our share of the flat. Please tell what can be done in this case. And will the sons get their share or not.