Revolution (make up brand) or MUA (Superdrug) would be a good place to start, affordable and good quality. Decent makeup brushes are always handy too, maybe a face and eye real techniques set to get her started.
At that age I learnt from YouTube, but I found a lot of videos are 1. American, so products are different. 2. Overly professional with expensive branded products and tools. So I would search things like “UK high street foundation tutorial” and the likes, so it’s relatable for her.
Also, figuring out her skin type, and using products that are made for that skin type is a must. I had really oily skin as a teen and would use Dewey foundations made for drier skin types, which just made me into an awful sweaty, shiny mess that smudged off and I always wondered where I was going wrong.
If she dry skinned, use cream products - oily skinned use setting powders with power products.
A little goes a long way, it’s a cliche but a true one. Foundation and eyeshadow especially, start with the tiniest amount, stand back from the mirror.. assess and add more if she thinks she needs it. But when your close up infringing a mirror concentrating on each part of your face when you stand back you realise you could have definitely used less and looked better doing it.
Let her experiment, have fun with it. You can’t learn if you don’t make mistakes.