All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If the situation is as you mentioned in the question, then your mother is obliged to repent to Allaah as it is forbidden for a woman to talk to a marriageable man in the manner which you described [in the question] as it is a means which leads to adultery. So, you should take the initiative to advise your mother and remind her of Allaah and frighten her with His Punishment. If you are unable to confront her with this, then you may write to her a letter informing her that you are aware of what she does, remind her of Allaah and threaten her that if she does not cut the relationship with that man, you will inform your father and your maternal uncle about it. It might be that Allaah will guide her and she will be deterred from what she is doing.
If it is possible not to let her go out except with your company or with one of your brothers, then it is an obligation to do so. We ask Allaah to guide her and purify her heart from the filth of desires.
Nonetheless, we advise you not to inform anyone else in the family about this before advising her, as it is an obligation to conceal the sins of a Muslim and concealing the sins of the parents comes in priority.
However, if she does not act according to the advice, then there is no harm in informing a member of the family, like your uncle or her husband [i.e. your father] so that he will deter her from what she does.
Finally, it should be noted that it is very important to have evidences proving such matters and not accuse someone -especially the parents- just out of suspicion or doubt.
Allaah Knows best.