Is it permissible for my mother to conceal the secret of a girl who is not a virgin, knowing that revealing her secret could lead to the ruin of a family?

The mother's action is fundamentally forbidden, as it is impermissible for her to look at the 'awrah (private parts) of girls except out of necessity, such as to confirm virginity. However, since she became aware that the girl was not a virgin, she did well by concealing it and refusing the money. This is further affirmed if the girl showed repentance and remorse.