As soon as your friend physically separates from her husband, she should go & get her benefits sorted. If she's working more that 16 hours pwk, she'll get working tax credit and child tax credit. Those are obtained via the inland revenue. If she's not working, she should make a claim for eithr income support or, since her kids are at school, jobseekers allowance may be more appropriate. Her husband would, under the CSA guidelines, be expected to pay 20% of his net income by way of child maintenance. He may also agree to pay her spousal maintenance for herself, tho' she may need a solicitor to get that out of him. If they have a mortgage and he won't pay, (or, if approp, rent),she should make a claim for housing benefit via the local authority. This doesn't kick in for ages, but she should make her claim immediately that they separate.
If she's unemployed or on a low wage, she may be entitled to public funding. Check out the Legal Services Commission website calculator.
So far as divorce proceedings are concerned, if she decides to go ahead now the only grounds are his unreasonable behaviour or his adultery. Assuming the latter is out, she should go on his behaviour. She (or you) can download the forms from the court website (www.courtservice.org.gov). She needs to complete the petition for divorce (if she needs advice on the particulars, let me know), the statement of arrangements for children and, if appropriate, EX180 (application for exemption from fees) or £300 cheque. She also needs to supply her marriage cert.
If in no hurry for a divorce, they could go to mediation and if successful, the deal should be embodied into a Separation Deed which is legally binding and can cover everything re. money, children,etcetc.
To find a local family specialist, try the national body www.resolution.org.com