Thanks Mercman, but I think you misunderstood the point I was trying to make.
I agree that it's foolish to underinsure - particularly when someone is going to be away from home for a long time. However, there is no point in insuring a risk that is not relevant e.g. if she's not going to be skydiving or snowboarding, there is no point in having cover for these risks unless the extra cost is relatively small. Likewise, there's no point in paying extra to cover travel in (say) Africa if there's no possibility that she will visit there as part of her trip.
The WorldNomads policy seems to be a 'one size fits all' policy that covers worldwide travel and various hazardous sports, etc. If there is something that meets her needs more specifically at a lower cost and without compromising the extent of the cover, then it makes sense to avail of the saving.
I would be very grateful for advice from anyone who has researched the types of cover that are available or who can suggest a website where these issues may have been discussed in more detail.
Regards
Homer