Your friend will have to give all her husbands documents to the solicitor and they will write to the asset holders and those to whom the deceased owed money to ascertain the size of the estate. She would be best to go and see the solicitor and take all such documents with her and he will be able to explain.
In terms of charges, most charge on an hourly rate. As your friend is the sole beneficiary and the solicitor is the executor he must write to her and give her details of the charges he is going to levy. She falls under the "third party client" rule of the Solicitors Conduct Rules and is entitled to be treated as if she were the client (on the basis that the solicitor cannot be his own client). Thus if she disagrees with the fees, she has the right to complain and request the Law Society to look at them afterwards. NB, this would NOT work if the solicitor was acting for a third party who was the executor since then the executor would be the client.