www.widowedandyoung.org.uk/bereavement-support/supporting-a-loved-one/what-to-say/
This link from the WAY website has lots of useful pointers. Do definitely encourage her to join WAY and claim the bereavement benefit.
One friend messaged me every day for a year with a good morning and a random comment. It helped to know someone was thinking of me.
If you have the capacity to help in the long term, be ready to go and give her a break further down the line. Do the washing up, the washing, cooking etc.. Make sure you are there for high days and holidays when friends will be with their own families. Have the kids overnight so she can have a night off.
Never assume she should be over it or tell her to move on. Even in 5 or 10 years. If you haven't been through it, you simply won't understand or get it right so just listen to her and believe her. My family haven't. Friends have been great and family mostly awful. Don't judge her because hopefully you will never get to understand.
Remember his birthday, death day, funeral date, their wedding anniversary and message her before she posts every year. All are hard forever. Make sure you sort Mother's Day for her until the kids are old enough to do so (or at least check a friend has and send the money). Same for her birthday and Christmas present from the kids.
If you can, I would go and stay nearby. It would be too much to have you staying (unless you can run the house until she can and she wants you to), but if you are nearby you can help - funeral outfits, readings, cars, flowers, music - it's like planning a wedding in a fortnight, but on your own when your head is totally mushed. Anything you can do to support her will help. The kids still need feeding, dropping to school, and there are so many calls to make.