This is one of the most awkward problems you can encounter in an estate.
The first thing they need to get is a copy of the will. They could try writing (or calling) local solicitors and asking them if they drew it up. I would be surprised if the original will was kept at home since normally that is kept at the solicitors and a copy kept at home (merely for safety's sake). They can check with the Probate Registry to see if it was registered there (there is a registry for Wills before death, but it is little used).
As for banks etc, most will want to see a copy death certificate and copy Will before they will release information. This is just going to be a slog finding out which ones.
Now as both executors are dead, a Grant of Letters of Administration (with Will Annexed) should be applied for (subject to an exception I state below). This can be applied for by either or both of the residuary beneficiaries. Unfortunately, one can apply without notice to the other. The Grant will be needed to release the assets unless the estate is below �15,000 and then will often be released without a Grant under the Administration of Estates (Small Payments) Act 1965. To use this latter procedure the banks will often require an indemnity but it is not beyond imagination that the brother will be able to persuade them to release the assets.
I think your friend should write immediately to her brother explaining that as a beneficiary to half of the estate and a potential administrator, she has the right to see ALL the information. I would actually advocate getting a solicitor involved now because if this guy has access to all the paperwork, it will be harded for your friend to get the Grant. There is a way it can be done (ie where you negotiate with the IR that you will provide an account of the assets once they have been delivered by the brother). Unfortunately, to get the Grant you need the original will.