It's so cool you are so supportive of your friend. Just that will help her loads. All the birthing books in the world won't do any good if she doesn't have the right people around to help her out. I know there are those who have done a twin birth UA, but it doesn't sound like your friend is going that way. If she is really trying for a non-surgical birth, she needs to find an attendant who knows about delivering breech. I had an OB that is older and very experienced with breech, so I lucked out. Baby A was vertex, Baby B breech, and all went well. A lot of drs won't go vaginal if one baby is breech and my OB wouldn't have gone vaginally if Baby A was breech, no matter the position of the other baby. Anyway, the point is, books aren't as important as finding the right birth attendant. I wish I had tips for that, but I don't. I just stumbled on mine.
As far as twin pg goes, the biggest thing to keep in mind, is that by the end of her 2nd trimester, her body will basically feel like she would at the end of her 3rd trimester with a single pg. BUT she still has that 3rd tri to endure!!!! EEEEK! That 3rd tri should be reserved for taking care of herself AND that is IT. No more outside employment, no trips, no running around all day long shopping excursions, no cleaning aka nesting marathons, none of that. Her body will be taxed enough. There are women that carry on like they would with a single pg and everyone and everything is OK. But why take the chance? And after the babies are born it is going to take all she's got to care for them (which she really should get as much help as possible for as long as she can), so it wouldn't be a good idea to wear herself out before the birth anyway. One thing I didn't realize when I was trying to plan ahead, but near the end of my pg I couldn't drive cuz my belly was too big, so I couldn't reach the pedals or the steering wheel. This is very common. Your friend should make connections with those that can help her with transportation needs for that possibility.
Nutrition is super important. Mostly it's about getting enough protein and fluids. An awesome prenatal like the Rainbow brand is essential.
As far as nursing goes, she needs to find a moms group for twins or multiples, or a LLL group and hook up with some moms who have successfully BF twins, esp those who have BF after a CS. That will be her best help. Also, an EZ2Nurse pillow will be a God send, whether she BF or not.
As far as being a mom of a singleton goes, try and keep in mind that while there are many things that are quite alike no matter how many babies you are carrying, having twins is NOT the same. If you can refrain from making comparisons, that's great. I doubt you would do this, but if I had a dollar for each time I had some woman tell about how she knew what I was going thru, yet she never had twins....So very frustrating. And try to take it easy on the "oh you have it so hard" comments too. If you keep things positive, that will be the best help. She already knows all the bad stuff, she doesn't need everyone else reminding her. Unless, like a PP said, she wants to talk about it.
If you are very close to this woman, it would be wonderful if you took the time and found out exactly what she is facing in terms of risks and possible outcomes and what not. That way, when she does talk to you, you are already clued in. I think the hardest thing about talking to those around me about my twin pg, is that none of them had been thru one, they had no frame of reference, so they had to have everything explained and/or didn't appreciate the consequences of the situation. It makes you feel very lonely sometimes. Ok, I wrote a book, sorry. HTH