It is very tough to say that she has started a relationship outside the marriage just because she changed her lifestyle so I would say you are not in a position to judge her based on her changes. Moreover, even if she starts affair you have nothing to say her as it is her personal life and you have no right to tell her how she has to live her life as besides your mom she is an individual woman too so she has a right to choose the best life for her.
Being a son you should ignore it as that is the best course of action for you. Does she start caring less about you or is she not a good mother? It would be better for you to consider her as an adult who knows how to live her life on her own. However, you can tell her that her new lifestyle makes you worry about her safety but you cannot ask her to change her life the way you like as it is not your life to control.