Just be there for her. Let her know you want to help. Offer specific things you can do, not just a general offer of help because it is hard to ask sometimes. Cook some meals that can go in their freezer, child friendly, so that there is a home cooked meal ready to chuck in the oven.
Offer lifts to hospital, offer to pick prescriptions up from chemist (the Bain of my life - I cope with big things whilst I look after my son with his brain tumour but I weep in the chemists as month after month there is always a delay with his prescriptions - I don not have 45 mins to waste in there).
Ring her often. Don't think 'I don't want to disturb her' if she doesn't want to talk she won't answer the phone but she will know you have rung.
Help to keep the kids lives as normal as possible. Bake cakes for them/with them, offer to take them swimming/soft play area.
Offer to babysit during the day so that if she feels well enough she can go out for lunch/walk/drive with her husband as she maybe too tired in the evenings to do anything.
Offer to do the housecleaning or pay for someone to come in every so often. Do some ironing?
There are lots of charities that offer holidays for families suffering from cancer - perhaps ask her husband if that would be something you could look into and research for them.
I am so sorry that your friend is having this fight again. My thoughts are with her and her family.
Cancer ruins lives totally and if you can help with anything her family will appreciate it.