Being a godparent is not just a title, it also comes with a legal responsibility, if done officially. (This also may mean that at 16 you cannot legally become god parents because you are not yet a legal adult)
The role of godparents, generally, is that if anything is to happen to both of a child's parents which makes them unable to care for the child (ie death, etc), then the godparents become the legal guardians of that child and are legally responsible for providing for that child.
If you do want to help your friend, by all means let her know that you're there for her, but also try and be sure she sees the situation relaistically and you do as well. At 16, who knows what can happen in your life (and also remember the fact that you and your boyfriend may not be together forever) and to tell your friend you'd be willing to accept that sort of position will force you to take a lot more personal responsibility (regardless of whether or not anything is made legal).
So, again, realistically, tell her you'll try and help her in any way you can, but also be sure she knows your own realistic limitations.