There is no inheritance tax on an estate passing to a surviving spouse, so she has nothing to worry about.
If the house was owned as a joint tenancy (as opposed to a tenancy in common) then she becomes the full owner by survivorship and the deceased's share of the house is not included in the estate so far as gifts to beneficiaries are concerned. However, this is not relevant to her as she is getting all the estate anyway. For the inheritance tax return, the deceased's share of the house has to be included, along with all his other assets, even though there will be no tax to pay.