Thanks so much for the great advice, everyone. I had suspected Chrome could be part of the problem, which is why I asked her about tabs - I know she has a habit of keeping them open. With all online courses at university this year, she ends up with a lot of readings and research in open tabs - I've introduced her to OneTab, which saves all the open tabs in a browser to be easily opened later with one click. She also ran CCleaner and went with all the recommendations it offered up.
Sad we can't upgrade the memory; I just assumed that was possible on every laptop these days.
I think the plan for now is she's keeping less tabs open and using OneTab when she needs to, and we're going to get on a video chat one day this week so she can screen share and I can have a look at her Task Manager and give her advice on what we can close, clear out of startup, etc.
In the meantime, she's been having a look on Task Manager herself. One thing we both thought was odd - Task Manager itself is usually at 10-15% CPU usage. I've found on my own computer, it might start at 5-10% for a couple of seconds, but then drops to below 1% and typically stays there. On her computer, it seems to stay in double digits. Not a big deal, but odd.
Thanks again for all the advice, and more is always welcome.