The intestacy rules are legally binding rules saying how everything that your daughter owned — her ‘estate’ — is shared out if she died without making a will. They will entitle her husband to a significant part of your daughter’s estate.
His entitlement would have lapsed if he had remarried or entered into a civil partnership, but you say he is still her husband.
If your daughter left children they may be able to go to court and claim that the intestacy rules do not provide them with ‘reasonable financial provision’ and ask for some of his share. They only have six months to bring their court claim. If he fights the claim, the costs will eat into your daughter’s estate, leaving less for everybody.
If the children are his, he may be prepared to give up all or part of his share in their favour. You should take legal advice to see whether you can negotiate an agreement rather than going to court.