Everybody's situation is different, but in general she needs to become knowledgeable about her and her husband's financial situation (income, expenses, property ownership/mortgage, and savings/investments), any benefits they receive and/or to which she may be entitled as a single parent, and she needs to get her legal position clarified. Once she has that information she'll be in a better position to understand what she can expect in maintenance payments and an eventual divorce settlement.
It's a matter of contention as to whether or not a SAH parent should start working before filing for divorce. The attorney my cousin saw told her emphatically that she should NOT increase her work hours (she worked one day a week) as it would decrease any support she was due. Other attorneys have told friends they should start trying to become self-supporting as it 'looks better' to the judge. Who knows?
In the meantime, if she can, she needs to start socking away cash where her husband will not find it. Ideally, it should be kept out of the house. Shave what she can off the grocery bills and do 'cash over' at the till. If he gives her an allowance, shave what she can off of that. My friend would buy something, say a small item of clothing or some towels, then return it and keep the cash. Look around to see if she has any personal items (not things that could be considered jointly owned) she could sell when the time is right. I kept this cash for a friend, we called it a 'fuck you fund', and I gave it back to her when she left.
She needs to start copying important documents both personal and financial, such as bank statements, deeds, wage slips, tax returns, and again, store them away from the house. Eventually she will want to move the original birth certificates, marriage certificate, passports to that safe place, but that will be immediately before she leaves. She needs to take a 'mental inventory' of items in the home that she may want to take with her such as heirlooms or mementos. If she chooses to make a paper list, it should be kept away from the home.
She may also want to take a 2-3 day supply of clothing for herself and the children, medications, makeup, etc and keep them elsewhere or hidden away in a 'bug out bag'.
A lot of this is written from the perspective of someone who has made the decision that it's worth it to leave if the husband can't be persuaded to. In most cases solicitors will advise her that it's in her best interests to stay put with the children in the marital home. But it's a long drawn out process to try and get him to leave, and if he hasn't actually been mentally or physically abusive she may not be able to actually make him leave. So it's always best to be prepared in case things become intolerable.