Why do poeple think adoption is such cruel choice? Absolutely absurd. Its the only compassionate choice in all this mess. Almost all of the time (esp in a case like this) the child will be going into a much better environment then the one it faced with its birth mother. Its not like they ship these kids off to Afganistan. I have had many freinds who were adopted, and an aunt of mine has adopted several children, and they all have wonderful families who love them dearly. Adoption agencies are very selective, and children only go to parents who really want them, and and have the ability and stability to provide a good home for them.
The only cruel choice in all this is for this 18 year old child to try and raise this child herself. She is far too young, inexperienced, uneducated, and emotionally and finacially unstable to be responsible for the creation and development of another human being. And so are you. The child is who this is really about. Not you or her and whatever feelings you may still have for each other. To see it otherwise is just selfish and foolish. You do not want to become a father, and what you are suggesting is just that whether you can see it or not. Think about it man. You want to move in your unstable pregnant ex-GF to live with you and your freind? I'm guessing you have never lived with a GF before. It is very challenging, learning how to live with someone. It takes a long period of adjustment, and comes with great responsibility and also a loss of freedom. On top of all that you want to complicate it even more by trying to bring a pregnancy and child into it as well, all the time trying to maintain a plutonic relationship with an exGf that you are playing father to the child of? And have your buddy there as well (what does he think of that? No way I would want to move into that sit. if I was your freind.) I won't even mention the real dad...
Imagine this: you move out and start your bachelor's dream; meet some girl and want to take her home. "Hey wanna go back to my place? Its a beautiful apartment. Just pay no mind to the girl living with me, she's just an ex. And if the kid calls me daddy, just remember its just a temp thing, I may work a second job for it, but its not mine. Hey! Hey baby, where ya goin'?"
Seriously though, if I sound harsh, I mean to be. You sound like a good guy who just wants to help out a girl he still has feelings for. Your heart is in a good place. However, the plan it has hatched, although it may sound good and noble now, won't be realized for the terrible mistake that it is until you get way too caught-up in this thing, and everyone will go though much more pain than they need to (there's enough already!) You've said that your done with this, but you are not, this thing is just starting. You're obviously stuck on this girl, and you won't walk away that easy. She's not going away either. If you really want to be a good guy and a true hero, get her to give the baby up for adoption, she's got nine long mos to think about it. Don't try and force it on her now, its a waste of time, things are way too raw and so many things can happen between now and the birth. Be a subtle and supportive friend and you'll be the hero that saved not only your life and future, but the baby's, and also hers in the end.
PS: Listen to Insidious. His words are wisdom Soybomb is also right, don't let her stay a single night, you must define the boundries and never cross them.
good luck buddy!