The answer really is - it depends. It depends how complicated the will is. If it just says " I leave everything to the cats' home" - no problem. If there are heaps of relatives, and he wants to create some fancy trusts, and there are lots of conditions - and above all, if it is an amateur will, you need serious advice.
In simple terms it means identifying and valuing his assets, applying for Probate, collecting the assets in and distributing them to the beneficiaries
There may be tax return(s) to do
Actually 'I leave everything to the cats home' can be a problem if he has relatives who cut up rough because they have been left nothing - and even if there are not, the cats home may be a problem wanting to scrutinise your every action in detail to make sure they've got every penny they are entitled to.
He's a retired accountant who never married so they wont be many relations involved. I'm pretty sure he'll have everything in the will down to a tee (remember he's an accountant) & they shouldn't be many probs...hopefully.