OP, please don't buy anything for your neighbour before checking what they would be happy to accept.
I had a similar problem with my neighbour who complained about cat poo (my window overlooks her garden and every cat in the neighbourhood visits it - I think the multitude of bird tables and feeders attracts them all) and asked me to restrain my cats.
I didn't want to be an inconsiderate neighbour so googled solutions and:
offered to buy her cat-repelling plants - she didn't want any of them growing in her garden;
sonic repellent - nope, suspicious of what they emit;
citrus peel - no, would look like litter;
netting - too ugly;
chicken wire - too inconvenient;
lion poo - it's just more shit really, innit?
She really just wanted me to keep my outdoor cats indoors, and didn't want to me to find ways to cat-proof her garden.
Thankfully, while I was going through my list in her kitchen, Tiddles from down the road appeared in the garden and squatted in an unmistakable manner. I asked what Tiddles's owner had offered to do and that's when it became apparent that, being closest, she had just assumed it was my cats (who have litter trays) and hadn't spoken to anyone else.
So please check with your neighbour and find out what other owners are doing about it before you part with any cash.