Would have been best to add your pics under your old question Julia, but never mind, I've checked back anyway - I'm not surprised Alan Titchmarsh doesn't know - I don't either, and I've been in the business for 30 years. I had an Aucuba that did something similar years and years ago, and it eventually looked so sick I dug it up. I didn't know what it was then, and I still don't know now, but I've seen it many times. It is not mentioned in my Pests and Diseases bible, and I'm annoyed that it seems not to be mentioned anywhere.
All that said (that's my moan for the day, well, first of many I expect, lol) I can see what looks like blistering on the black leaf in the top pic - is it possible that the plant has been exposed to weedkiller or similar, say from spray drift (fields nearby?) or accidentally by your friend - has she sprayed weedkiller anywhere at all?

I think that this could well be an infection. I read a year or two ago about the possible causes of this problem and it could be one of two things perhaps in combination.

It can be caused by bad drainage which becomes apparent after a really wet winter. Some of the roots start to die back and this begins to cause problems at the top of the plant such as this blackening and die back. I wish I could find the site that said this but cannot seem to find any more. If I do find it I will add it to this post.

The second possibility is after really cold winters, the bark at the base of the plant or at the base of a primary branch can split open allowing infection in. Whichever the possible cause, it appears that the winter time is the problem ie one that is very wet or particularly cold or combination of the two.

Though Aucuba are effectively evergreen Cornus (Dogwoods), they don't tolerate saturated soils for very long and quickly succumb.

Thanks to all, this is a bit of a mystery isn't it? For information in case anyone else gets this, my sister in Houston also had this and thinks the problem was LACK of water! My friend has another couple of these and they are showing a small amount of black leaves. I'm going to have a good look and prune the black off and burn it, and wait and see what happens. The RHS say it's water logging to, but her garden is the dryest I know. ODD!! Thanks again everyone.
xx