If he wants to record his fortunate sessions via twitch or edit video and upload to YouTube, he will need something more around £800-1000 mark and even that will be a frustrating experience.
You will want a PC or gaming laptop (I have a laptop these days but used to build my own) that has minimum:
8GB ram
An SSD type hard drive, min 250GB but more storage for creating video- could save cost by having an external drive for that.
An Intel corei5 or above processor (cpu).
Or the equivalent AMD processor (I'm not up on the latest models)
A Dedicated graphics card of at least 4GB but probably 8GB these days? Someone can correct me here it's been 3 years since I last upgraded. I'd have thought 4GB is fine for fortunate but maybe not video editing?
When buying, look out for DEDICATED graphics, you do not want "on board" graphics. You need a separate graphics card, not one build into the motherboard. Many machines have both, the on board runs Windows and the dedicated one spins up for gaming (and gets noisy).
I doubt he needs 4k graphics, and on your budget I would just go with standard full HD at 1080p with screen to match.
Anything else can be ignored really. Unless you are building your own but based on your info given so far I would not recommend it unless you know someone who can spec it and do it for you.
He may also want to invest in a decent gaming mouse and keyboard, these can set you back a couple of hundred if he wants to be competitive. It's tricky as it's quite an investment, but can always upgrade if he gets into it or saves up his pocket money. I got a part time job when I was 15 (20 years ago!!) And saved it all up to buy my first PC from dixons, some crappy thing for about £800! Learned to build my own and got much more for my money in later years.
Now I'm old and lazy and earn real money so gaming laptop it is, i got a Aorus G3 for £1200 which now appears to be a bargain. DP has a fancy Razer laptop which was 2k, much better than mine.
But, he should see how he goes. He won't need that for just fortnite!
If you are totally clueless, start by talking to PC world, but do not buy their overpriced rubbish without advice from a proper techie (sometimes they have something ok). Then look on Overclockers website to learn what is good value.