If you hadn't, she might well have had far more difficult and messier choices to make. Better safe than sorry. Not that she is yet entirely safe. You must try to persuade your stepdaughter to visit her GP or one of the special clinics as soon as possible. She won't be read a lecture and her parents will not be informed.
Total confidentiality is guaranteed.
Even if she says she has no symptoms, it cannot be stressed too often that not only is it possible to be infected without any obvious signs, but that STDs can have far-reaching consequences, including making women infertile. The next point is whether you go behind the backs of your husband and his ex-wife and pressurise her into taking the Pill. Some might say you should just order her not to have sex - but with something like 40 per cent of kids these days regularly having underage sex, that isn't always realistic. Fortunately, your stepdaughter trusts you, so I think you should advise her that if she's determined to be sexually active, she must not risk an unwanted pregnancy again. That means contraception - but whether that is the Pill is another question, largely because of what I have already said about STDs.