How do you tell a parent in denial that their teen daughter (16) is ually active?

my son is dating the girl and he is a little older. her mom says she knows her daughter and that the girl will tell her everything that she always has. well i confronted the kids and asked the girl when she planned to talk to her mom about birth control and she said NEVER.. her mom has tendencies to punish by removing cell phone and pc privileges pretty much cutting her off from the rest of the world. how do i help the girl and not undermine her mom? she needs to have the talk with a professional unfortunately in our state she has to be 18 to be seen in a clinic or by a doctor. any advice would be appreciated.