It depends when he served. Before 1 April 1978 there was a rule which required that the couple should have been married during the period of military service.
Couple of examples:
So, if someone served from, say, 1970 to 1982 and was married before or during service (and still married to the same person when they died) the spouse would be entitled to a pension based on the whole service.
If someone with the same period of service married or remarried after they left service, the spouse at the time of the member's death would be entitled to a pension based on service after 1 April 1978 only.
Does that help?