ABSOLUTELY NOT!!!! It would be an amazing thing you did.
Really though, its whatever is in your best interest. I have Cf as well and I am 22, but there is times where I feel so down that I cannot do anything for myself, never mind a young child. It depends on how your boyfriend would feel about taking on such a responsiblity. Do you feel like you could handle this sort of thing? You would be putting yourself in a financial bind to help out your friend. Its totally up to you. I would say go for it but unfortunately I do not know you financial situation, nor do I know the kind of friendship you guys have. Also, I dont think you would be making a stupid decision by giving up your job.
To be honest, three weeks is very normal for someone with CF, so if its happening now, it will only become more stays from here on and I am sure she would appreciate it more than anything, but are you able to do it. If so... GO FOR IT!!!!
best of luck... at least the little guy will have a great life.. two families who love him dearly.. in the end its really all about the child and what kind of life he would live if no one could keep him..
Anyways.. I really hope that my opinion helped.