So he sold a building & granted a mortgage to the people who bought it. They have been making payments to him under this mortgage. The mortgage states that on his death they should make payments to his daughter.
I assume this is what you mean - if not can you please clarify.
I assume he died intestate, because the death bed marriage will have invalidated any previous will (unless made in anticipation of marriage which seems unlikely given the circumstances). So his widow will inherit the amount the intestacy laws state.
The question then is whether the benefit of the mortgage is part of his estate - if it is, then it goes to the widow (unless the total estate is above the intestacy amount). I think it is almost certain it will be part of the estate, but your wife needs to see a solicitor with the mortgage document to get definite advice on it - the outcome may possibly depend on the precise wording of the mortgage deed.