I will use the child theme, as it’s possible that at some moment I may need to go into the files, yes. In that case I see Jetpack more as a convenience for my friend afterwards, as she gets just the one option in the menu clearly stating ‘Edit CSS’. And possibly Jetpack has some other things that’ll be of use for her too. I’ll look into that.
The menu is custom, so far, in that I started changing colours, and I’ll start changing width, position and a couple of other things. When I tried to change colours in the stylesheet I discovered later that those changes regarding options accessible through ‘Theme | Customize’ do not work, so I’ll have to change things in two different places. Not a problem, as it’s logical once I think about it. As you say, I’ll have to test Jetpack and see whether it works the same way.