I am not sure if she is planning on telling the other parents. I know that you said she mentioned it in confidence to you. My mom had cancer and the treatment made her very tired. If some parents could be sure to volunteer in the room or come in just to get some "prep work" done for her - such as cutting out shapes for activities, making examples, checking papers, straightening shelves, etc. I am sure that would be appreciated. I am a teacher and know how much time all of the "behind the scenes" work can take.
If you want to do meals, I would check to see if she is having any cavings, or repulsions from certain foods. Treatment can change the way that things taste for certain people and a dish they would normally like may no longer be appealing. Perhaps meals in small portions wouldbe good so she can freeze some for future days. Or perhaps have a rotation schedule where maybe paretns sign up to make a dish on Monday, Wednesdays and Fridays maybe.
Just keep asking. Sometimes people are hesitant to accept help the first time it is offered.
It is very thoughtful of you to look out for her :)