Boyfriend mother up to something

She still talks to his ex and constantly brings her up to him knowing that he has a new girlfriend . She has met me several times. He gets very upset when she does that especially since the girl is the one who cheated while she was pregnant and that broke his heart . He keeps telling me that he isn't paying his mothers comments any mind. In the back of my head I just don't understand why she keeps bringing her up . A small part of me is hoping he won't start thinking about her again . I don't know why his mother is doing this