Layman here but surely any property focused solicitor will be able to investigate for him. Sounds a bit bizarre he never investigated further. Who is managing the properties now. Does he not have any paperwork?
He needs to get a copy of the will, then he needs to get land registry docs for those properties. Is his dad still alive? There must have been something in place when she dies - properties put into trust/someone put I. Place to manage the properties?? Does he know which properties they are eg does his Dad/nan/sibling currently live in them so it’s more a case of just proving he is the owner or is it he has no idea which properties so needs to find out what they are, who lives there and who is registered as owner??
If his mother had properties to leave the surely it will have gone through probate. A will is a public document once this has happened, anyone can get a copy. He can do it himself, no need for a solicitor.