This is her third baby, this Momma needs a few sets of extra hands at this time in her life, even if her new baby was trouble free. By all means, make meals and deliver them. Make things ahead and freeze them for her, so when she wants to put the meal on the table she can pull it out of the frig or freezer to cook and serve.
Help with the laundry. Take her other two kids to a movie, rent a movie to show at your house to entertain her older kids and yours. Make sure that things are ready for her to bring baby home. Ask if there's anything you can get ready for her.
Go get her a bubble bath, a nice shampoo and conditioner, body lotion... to help her unwind and be ready for the next day's list of things to be done. Most hospitals have a chair side massage vendor, get her a gift certificate for one. It will help her relax and let go of some of her tension.
She's spending her days at the hospital, record her older kids on your cellphone and send it to her, be it a video or multiple pictures. So she can see that they're okay. A journal and a nice pen for her to write down her thoughts as she watches over her baby. It will give her place to vent and reflect.
I would not go to the NICU with any type of sniffle, sneeze, cough, ache or pain. These little people are fighting for their lives in a sterile environment, adding adult size viruses is too much for them. Besides, with all the nurses, docs, immediate family members.. that room gets pretty crowded.
Above all, don't forget Dad. Dad is going thru many of the same emotions and tensions that Mom is. Dad's believe they can and should keep harm away from their family and this was beyond his control. Dad's need some TLC as well. So the meals, the babysitting, kid activities will be quite helpful, useful for him. But he also needs an ear, a shoulder, a friend... and as much as you may think you can do this, it really helps if he can vent and talk with a man-friend. If this is your husband, then arrange for them to have some one on one time without the kids and you around.
This is a difficult time for them, their emotions are all over the scale. Thank you for being a great friend to this family.