Both my parents went through lung cancer and both had Chemotherapy. It's tough to watch them go through it. Just being with them and checking on them is a big help. Making up a few meals and putting them in the refridgerator so she only has to warm them up would be a giant help. Everyone handles chemo as different as the types of chemo that exist but my Mom was too weak to cook while getting treated. A friend of mine was going through chemo a few years back and never missed a day of work, so everyone is different. The hardest day was when my Mom lost her hair, although she knew ahead of time it was going to happen, it was way different when it did happen, that was a pretty emotional episode for my Mom and I was glad to be there for her. Good luck and I hope her treatment works.
Never ever give up.