As far as I can see from various bits of guidance on the web, a War Widow's Pension (whilst called a pension) is actually classified as a benefit and is not a pension entitlement as such (in the same way that the basic State Pension is).
If that is true, if she is entitled to a State Pension, there is no reason why the War Widow's Pension should not also be paid. If the combination of State and War Widows pension is still short of the minimum, then Pension Credit (or other benefits) could also be claimed.
The War Pensions Agency is the place to get a definitive answer on 0800-169-2277.