Wow...this is always a tough call. Normally, I am not an advocate of adoptive families lending or giving money to birthparents (either before or after placement). I agree that it can set a difficult precedent and make things awkward. I feel even more odd speaking up here, because another mom asked for opinions on this subject (different circumstances, but still...) recently, and I advised her not to say yes to her child's birthmom's request.
HOWEVER...Based on what you've told us, and how urgent the medical need seems, I agree with Jen in this case. I would say "yes", too.
If she already exhausted her family as a possible source of help, she must have really been feeling awful having to call you in the first place. I know I would have. So I'd just proceed as you already are: treat this as an emergency, one-time request from someone very close to you. Act and think as though it will not be repeated, and gosh, I hope for your sake and hers it doesn't ever have to be.
I'll keep bmom in my prayers and hope for her quick and smooth recovery. You and your family are in my thoughts, too. Good luck.