I don't see that it could hurt at all - it's worth a try! If it's any help to your cousin, I had the same problem with DD1 - I had a severe PPH and just didn't produce any milk. Second time around, I had no complications and my milk came in straight away, so just let her know she should be fine :)
I have also heard that it's possibly to stimulate a milk supply before baby's birth, and these cookies are very inexpensive to make (I scored a huge bag of linseed/flaxseed meal for under $5 and it will last me for months!, and the brewer's yeast is about $1 a tbs so it's not cost-prohibitive), so if it doesn't work for her she won't be too out of pocket :)
Wish her luck for me!!

