If your gran left a valid will, only the person(s) named as executor(s) in that will can be granted probate.
However if she died without a valid will there's a 'pecking order' of people who can apply for a grant a letters of administration. Any of her children could apply for the grant, with none of them having priority over their siblings.
If you're sure that your gran left a valid will (and that the will definitely named your father as executor of her estate) your father should contact the probate registry to query the grant of probate. You can find the relevant contact details here:
http://www.hmcourts-service.gov.uk/infoabout/c ivil/probate/registries.htm
Chris