robinw, i'm not an expert but this doesn't seem right to me. the only thing i can think of is that the fact that your neighbour lived in the house after her husband's death means that for iht purposes the whole bequest is treated as having gone to her - which would as you say have been exempt from iht. then on the second death it is treated as the bequest to the charity and to the daughter in which case iht would be payable. i think taper relief (i.e. whereby the tax decreases depending on the length of time since the gift) is only relevant for gifts during life.
anyway, if your neighbour's daughter is unsatisfied as to the explanation she has received, she should most certainly get a second opinion.
hth.