In reply to ben b:
The basic TomToms that have a touch screen and plug into the ciggy lighter are good enough. The only problem I can see is that it isn't obvious from the initial screens how you change things, such as the volume and the map view: you have to just touch the various icons and go into the menus and trial by error you figure out what does that. Therefore, confusion might lead to impatience and the only recourse is the manual, which is probably a freakin download, though it may be a very basic booklet, I cannot remember.
However, if the thing can just be left at the right volume and turn by turn voice directions and the basic map view, it may be possible to set it and leave it alone.
Does the neighbour have anything similar? Might be worth finding out as that gives your relative someone to go to for basic advice; though they could swap it out for a good old fashioned A to Z when you aren't there