She should talk to the mother and know what is really wrong to talk to a boy. That way the mother will be able to give her fears of not letting the daughter to talk to boys. The main reasons are that, the boy would lure the girl to bad morals such as premature sex or abuse of alcohol and drugs. Maybe because she already knows the behavior of that boy and as you know bad morals ruin good morals. The girl should displace good character and up hold good morals. Also she should try to win her mother's trust and belief that she can be responsible.
Well, most likely your mother has a reason for not letting you talk to a certain boy. I think you should honor your moms choice and not talk to the boy. It will save you a lot of trouble you don't really need.
Is it one certain boy or any boy? How old are you? If its one boy could be she has a reason. Talk to her maybe let her meet him. If any boy. Talk to her find out why. She sounds like she's just protecting you. You have to get her trust when comes to boys. Don't forget she was your age once and sure she remembers how boys were. Trust me they haven't changed much.
I agree with cookie and Tony... Listen to your mother. She has a reason and I can tell you honestly that in every single case that my mom disapproved of someone I wanted to talk to or hang with she was right.