Whatever she asks for! The father of my baby girl brought my favourite chicken and bacon vienna to the hospital (I couldn't stomach the food there!), that was enough for me :-).
Kind words are the best gift though. Tell her what an amazing job she did and how wonderful a' mother she's going to be!
You haven't said if she's at home or in the hospital, or if the baby's your's or just a friends'....makes a massive difference!
Either way, everyone brings gifts for the baby and I think the mother deserves something too. What would be the best gift for you if you'd just experienced 48 hours of true agony and felt every emotion all rolled into one?