Go to a mediation service I think the CAB can help.
As you say solicitors fees are huge at least £50 a page just to write a letter. I know this from my daughters separation.
As to the mortgage they need to contact the mortgage provider, even if as you say the husband can afford the payments on his own they need the lenders permission to make the change. The lender may not be happy as they are losing the security of having two working people to share payment and may not want to change to a single person paying. As it is a joint mortgage it will have to be replaced by a single mortgage anyway.
If the mortgage lender will not accept it , that changes the entire situation. So get that sorted first!