Sharing such stories is completely inappropriate and violates privacy and social norms. This is a very private and inappropriate topic that should not be publicly discussed.
This is a very difficult and disturbing situation. You must act quickly and assertively. Start by having a private talk with your mother. Express your extreme discomfort and make it clear that this behavior is unacceptable. At the same time, be there for your girlfriend. Listen to her feelings and offer her all the support she needs. You may also need to consider involving a professional counselor if the situation is too complex to handle on your own.