This is a difficult situation both legally and emotionally.
Depending on the exact age difference of these two, the boy could be committing a sex crime, if he indeed was sexually involved with this girl and at least 48 months older than her. That is something to consider, I am no lawyer, but he may want to consult one before getting more involved.
Another point is to make sure these young people are educated enough to know how to protect themselves from sexually transmitted diseases and unintended pregnancy.
It's not too unusual for teenagers to engage in promiscuity as a reaction of problems at home. This girl obviously does not feel like she is loved and/or welcomed at home and is looking for attention elsewhere. You could urge her to speak to a counselor at school or talk to her mom about some professional help for their problems at home.
To be quite frank, most of my friends lost their virginity around that age (between 14 and 16) with no apparent harm to them neither back then nor in their future. Of course this was not in the US, but in my home country premarital sex is generally tolerated and children receive sex education as early as elementary school. All of my friends were on the pill before they started having sex, it was a pretty common topic. Most of them are now married, or in committed relationships and have no regrets about the past. We often talk about how our former boyfriends might be doing now, there is always someone who ran into someone...
In my eyes, fact is, that she is probably going to end up having sex sooner or later (probably sooner) and your main focus should be that she practices safe sex, when she does.
It's a difficult situation for you, since you are not her mom. You can't actually forbid her to see the boy or take her to get health services - but you can show her where to access that information (for example planned parenthood, or even her family doctor) and communicate to her that you would encourage her to wait.