Has she got a birth plan for her C/S?
I would think that the most important thing you can do for her would be to make sure that the medical team follow her wishes both during the procedure and in the days afterwards.

Is she planning on breastfeeding? You can help her by making sure she can have skin to skin contact with her baby as soon as possible and that she can have her baby with her in recovery. Also for the first few days it will be hard for her to lift her baby so being there as much as possible for her without crowding her would be great.

You're very lucky to have such an honour. From your desire to do it well I can see why you've been chosen.
Good luck