As an 18 year old girl, it is unlikely you would be considered as a viable adoptive parent. However, the good news is that nearly EVERYONE has ovarian cysts at some point in their life and having them does not mean you cannot conceive a child! So there is the good news.
I'll go along with what everyone else here has said, you need to tell your friend's parents she is pregnant. If she has known since December, then she is at the very least 6 months pregnant - which would be about right, considering the fetal movement.
Sounds to me like she might try to get rid of the baby anyway she can. Doing drugs, pre-term labor, etc.
More than likely, you could report her to CPS, and they'd not do very much. Please do, though. And also her parents. Whoever is supporting her needs to be involved.
Adopting a drug addicted baby is probably not something you should consider at this point. There are plenty of families who are more suited both emotionally and financially for this kind of task.
Good luck!