Husband left her

She should look for a local Law Centre and /or find out if a local solicitor offers free advice. Some will give half an hour.

I believe most solicicitors will give a half-hour free consultation where they will advise you. Obviously they won't act for you without getting paid for their work, but they might tell you what your legal position is, and what your best plan of action might be.

I think gingerbread have a helpline, and there is the citizens advice bureau.
Is she aware of the benefits system, and how it works?

Thank you, unfortunately she is so much in debt she can’t afford solicitor

Need a solicitor and also look at Housing Benefit.

Yes, she needs to see a solicitor who specialises in separation/divorce issues. An internet search will help her find a specialist. Most good firms will also have a mediator who can assist in discussion about contact, residence and financial contributions.
As trisher said, it will help if she sets a boundary around the things she discusses with her mother in law, to avoid a fall out. What a tough time - legal advice essential but legal aid no longer available shockingly.

She needs a good solicitor. Tell her to ask around if she knows anyone who has had a good divorce settlement or go on line and do some research. She shouldn't just pick one out the phone book. They will need to go to mediation anyway where they will discuss financial matters and he won't get everything wants. He will have to keep a roof over his children's heads. She should talk to the mother in law but try not to discuss his behaviour as it's something they will probably never agree on.