Brilliant, really useful advice, I will have a look at those options.
I only have a basic knowledge of Raspberry Pi, but I am aware that you can get a variety of add-on modules including GPS (useful if you want to move it around outside), bluetooth and phone links. You can set up a reporting interface that from your mobile will give you the trend for however long you want, in graph form and showing thresholds.
This can then be private for you, or you can share with the neighbour so they can see what impact they are having.
You can also then set up messaging and alerts that can text you and your neighbour when it goes over whatever threshold you set.
While I am aware that these things are possible, having seen a friend build one of these systems, I haven’t done it myself. I suspect that the people I know who do know these systems would put this together in an afternoon. Starting from scratch would be a couple of days if you got support from either friends or the right forum; if just using books and YouTube instructions it might be a week or two’s work.