I've been a gamer for 20+ years.
At the start of last September I bought a bleeding-edge Alienware laptop for £2500 with ALL the trimmings. We're talking way beyond what any game needs in order to run smoothly.
If she's looking at buying a desktop, it should be less than £2000 for a very solid one.
She should be able to get an i7 Processor, GTX1080 GPU, 16GB of RAM and a decent SSD + HDD all for under £1500. Does she have a monitor already?
Pre-built gaming PCs always cost more than components and generally aren't worth it. Putting a PC together is very straightforward these days because a lot of the circuitry is a heck of a lot more robust than it used to be 15 years ago, and thus more forgiving of mistakes.
Encourage her to shop around and consider buying the parts and assembling it herself, rather than ordering a full PC. She would probably save about £300 by doing this. There are hundreds of youtube video tutorials showing how to put a PC together if she wants to learn.
That said, if she's been disciplined enough to save for years and years into the 2 grand territory, I'd say she's earned it. A PC this expensive will also last for a minimum of about 5 years, and even then, a couple of hundred quid in 2022 will let it last another 2-3 beyond that. Considering how many hours she's likely to sink into using it from 14-20, that could be worse.
Ask about what games she plays, and what the recommended specifications are. Point out that she's going to be able to drive in 3-4 years time and might want to think about keeping a little money back for a car, etc etc.