1. When You're Expecting Twins, Triplets or Quads by Dr. Barbara Luke and Tamara Eberlein. Her nutrition program is unparalleled. Also check her website at www.drbarbaraluke.com
and suggest her highly recommended nutritional assessment service. Worth more than she charges I can say from personal experience.
2. Having Twins by Elisabeth Noble. Out in many many editions. A classic.
3. Mothering Multiples by Karen Kerkhoff Gromada pub. by LLL. The go-to resource for breastfeeding and mothering multiples. Also see her website at www.karengromada.com
, worth it for the photos section alone. This book is set to come out in a new edition in September, so the current edition may be available already at a discount some places. Worth getting it now and then getting the new edition, it's that good.
Congrats to your friend!