It all depends what his Mum’s Will says.
Most of us who did wills with the advice of a solicitor have clauses that say what should happen if one of the beneficiaries pre-deceases them. Typically that the money passed to their children.
If this is the case, yes, if something happens to your Dd Dad then the money would go, as specified in her will, direct to your Dd.
If she has just said ‘my children’, and your ex has died leaving a sibling, then the money will go to the sibling.
Nothing your ex puts in a will will affect what happens wrt his mother’s Will when she dies.
He needs to make his own Will leaving his estate, whatever it is and whether or not he inherits from his Mum, to his Dd.
If she is in a home, has a charge been put in the house to pay for it? That will be paid first before any money goes to beneficiaries.