However well you know your neighbour, I don't think you can possibly mention this problem to her without causing offence, but there is quite honestly no other way of solving the problem.
You might, just might, get away with asking if they use the kind of litter that clumps when wet or the other kind.
I have always used the kind that clumps, as it makes it possible simply to scoop up the clumps every morning, when I attend to the tray.
But, I have two cat trays - one in use, and one emptied, washed in soapy water and dried outside in the garden, and the tray is changed completely once every ten days, at the moment while the current felines are kittens. Later on, if they don't go out into our garden, as I confidently expect they will prefer, it will be changed once a week.
A cat tray smells if not changed often enough- how often that is depends on the cat to a large extend. Obviously an unneuter adult male will smell more strongly than neutered cats of either sex, or a female.
It surprises me your neighbour can get away with this -all my cats have refused point-blank to use a dirty, smelly tray, and have either sent me filthy looks until I changed it, or wet the floor beside it!