Hi Hash
I suppose it depends on the sort of editing she will be doing. Most editing software packages employ "non-linear editing" - you upload your footage/media and then tell your computer which sections files you want to appear at a particular point on a time-line. Your computer will then fetch that part of the file and make it appear at the relevant point, then fetch the next file and so on - the trick is for your computer to make this appear to happen in real time (most editing software are extremely good at this and make it seem almost effortless). So the important factor is how much work you need your computer to do - how many files, how many sections of files, adding transitions and effects etc all add to the workload.
If she's just sticking a few clips together then any PC will do the job with some 'free' software e.g.
https://www.nchsoftware.com/videopad/However if she's doing more serious stuff then you need to look for a machine with a reputable processor with as high GHz as you can get/afford (Alty TV's has 4GHz). You'll also need plenty of RAM (we have 24 GB but I'd recommend at least 16) and as good a GPU as you can stretch to.
When I started to dabble in video editing about 15 years ago PC's were absolutely crap at the above and would frequently crash and drop frames. Lots of folk tell me, with the right spec (see above), PC's do a perfectly good job now however I'm firmly in Mac camp when it comes to both software and hardware. Mac's are built for handling media whether you're using a Macbook or an iMac they'll all handle video media easily (but again higher spec processor/RAM/GPU might be considered). As for software the two big boys are Adobe Premiere Pro and Final Cut Pro - both great bits of software. For me it's always an iMac/Final Cut combo but with if you'd prefer a good spec PC I'd recommend using Adobe Premier.
Hope this helps but let me know if you have any questions.