Pants wise I'm a size ten usually and had a size 16 midi pants from Sainsbury's as they need to come really high to not be anywhere near scar/bandage but also pregnancy tummy doesn't go down immediately. I'd really recommend the Sainsbury's ones as they were cheap enough to throw away but nice and soft.
Wind-eze for the trapped wind is essential, and abdominal surgery causes pockets of trapped wind and it's very painful
Also movicol of fyba-gel sachets not lactulose as that will cause cramping. Absolutely needed as abdominal surgery caused a sluggish bowel plus the pain relief causes constipation.
I was ridiculously thirsty after my section and a friend had told me the same so I'd taken a big water bottle. I drank litres of water afterwards. So would recommend a sports bottle of water, so she can just pop it next to her.
In terms of recovery, I had an emergency section under general but was up and walking a bit 12 hours later, she needs to move around to avoid DVT but not over do it. A few times I did too much and set myself backward pain wise for a few days.
You may find you need to pull her up from lay down, as your core feels completely gone!
My husband did all the nappy changes etc for the first two weeks so I didn't have to bend over, he would pass baby to me from his basket and so on as you shouldn't be lifting afterwards. I wandered slowly around the house, but didn't carry baby up or down the stairs for the first week as I felt I needed to hold my wound when walking.
Also tell her to keep her wound super dry, I used the hairdryer on mine after a bath to ensure it was properly dry and it healed really well and quickly. Often infections start from wounds not properly drying.
It sounds like you want to do everything you can to help, which is lovely.
Good luck to you both