First question: in whose possession are the rifles & shotguns now? If the deceased dad was the licence holder, then the weapons will now have to be either with an RFD (registered firearms dealer) for safekeeping, or the Police. If its the Police, the weapons may already be scrap, depending upon the circumstances of the case.
Second question: if the firearms still exist, is ownership proven by an executed will or other document? If not, the firearms will not be going anywhere.
If the weapons are in the possession of an RFD, then they should simply be passed to a specialist firearms exporter/freight forwarder. There are a few that specialise in the (careful) packing of guns for export.
For a non-resident foreigner to try and get a temporary certificate to hold the weapons is probably going to be very complicated and time-consuming, even if its possible. Normally, temporary certs are issued to those bringing guns into the country on a temporary basis (eg sportsmen). I have to confess I do not know about temporary certification to acquire and export guns.