Oh, and I'd buy the huge Sam's Club size boxes for the amount I listed (so 3-4 of the size 1 in the Sam's Club box; not the Walmart size).
For me I am buying 2 packs of newborn (my girls had big booties and lots of chunk so they went through them quick), about 3-4 of size 1, 3-4 of size 2, and 8 packs of size 3. I'm also going to stock up on Pampers Sensitive Wipes (seriously the best wipes) and I think Huggies are the best diapers.
I buy my diaps from Walmart so if an allergy or sensitivity does pop up I can exchange the unopened diapers for a different brand!
Tell your friend to stalk up on diapers at Costco or Sam's. The boxes have multi-sized diapers.
EVERY baby is different. My son SAILED through NB size and even size 1. He's in 2's now and is only 2 1/2 months, but my daughter didn't grow as fast when she was a baby. Get a good amount of each and if you have extras of one size, they always make a great gift for someone else who is expecting!!
It's great to be prepared, but I learned from my son to wait. I went out and bought several boxes for newborns, and had to give them all away because he was allergic to them, and could only use cloth diapers until he was about 9 months.
Don't know how common that is.....
Honestly this is going to contridict what ashiepooh says but I would buy like 5-6 if not more packs of newborn diapers. In the beginning babies will poop their diapers like 10 times a day so you go through diapers extrememly fast but they grow out of them fast as well so she's probably going to want to double up on size one as well. Then once she gets to size 2 and 3 she can buy more because they will stay in them longer but use them more slowly.
2 to 3 packs of NEWBORN
2 to 3 packs of SIZE 1
4-6 packs of size 2
4-6 packs of size 3.....
If she buys more of the smaller sizes, tell her to keep her receipts because babies grow out of them super fast!
Tell her to start buying wipes as well, because when a baby get diarreha you'll go through wipes fast! Plus it's better to stock up and be prepared!
Ultimately, any left over diapers from a pack, she can always give to someone else she knows, who is going to have a kid or has kids younger. I did that with a few of my friends, i'd have left overs of an open pack and hand them off!