IHT is a tax levied on the transfer of value as a result of death so funds outside the estate in which she had an interest will be included. On the widow's death, the half share of the estate in which she had a life interest is aggregated with her free estate for the purposes of IHT. IHT is then allocated proportionately between the free estate and the aggregable funds. EG say the widow owned assets of her own amounting to �200,000 and the value of the fund in which she had an interest was worth �200,000 the value of the transfer would be �400,000 and the resulting tax would be payable out of the free estate and aggregable funds in equal shares.
If the widow's death comes within 2 years of her husbands I would suggest that legal advice is sought asap since it may be possible to vary the husband's intestacy in order to make better use of the nil rate bands and thus save tax.