@sunglasses Just a few thoughts. You said:
"... [the solicitor] ‘can’t believe her husband made no provision for her, severing the will was an ill advised thing to do, she should challenge it and get the house completely in her name’ etc. At that point the solicitor hadn’t seen a copy of her husbands will"
OK this is all very confusing. The solicitor said this WITHOUT seeing the will?
Where you say "severing the will" (the phrase is "severing the tenancy") that is certainly not an "ill advised" thing to do at all. "Severing the tenancy" just means changing from joint tenants to tenants in common (you "sever" the joint tenancy and replace it with a tenancy in common).
This is quite commonly done in the case of blended families or concerns that the surviving spouse may remarry and not leave anything to the existing children. It is also common where a couple is concerned about care home fees (if one of them were to end up in a care home) wiping out the entire inheritance for their children.
"...‘can’t believe her husband made no provision for her..."
This would happen where mum had NOT been made a life tenant of the property. Although how the solicitor would know this without actually seeing the will, I have no idea.
This is why I said that you really do need to read the will and see what it actually says. If his mum was made a life tenant then that means exactly what it says and (except for some very unusual situations) she cannot be made to leave the property.
However, if "...her husband made no provision for her..." that would imply that she was NOT made a life tenant. But how the solicitor knew that without seeing the will I don't know.
In that situation then the sons could only remove mum from the house if they get a court order to sell the house.
"...he is now charging her to register the house in the 3 names. It seems this might be unnecessary as the Will states clearly that his half of the house has been left to the two sons and she retains her half..."
You are correct. Doing this will not give his mum any extra protection beyond what she already has.
"...he is now charging her to register the house in the 3 names."
That really does sound a bit dodgy. There's no need to do that, but if you really did want to do this then you can just fill in the Land Registry forms yourself.
If his mum really is worried that her sons might chuck her out of her home because the will does not say that she is a life tenant then the other thing to do would be for the sons to offer her a life tenancy of their share of the house.
This is known as a secured tenancy, and is a life tenancy (like the old council house tenancies). This is straight forward to set up but it would be best to get a solicitor to advise (although maybe not the one that seems to be frightening his mum!). Perhaps go to a different solicitor if you really want to go this route.