Basically, you're going to struggle to get any kind of PC for under £300. His friends are talking rubbish that you need to spend a grand, but you going to be looking at about £600 when you include all the peripherals like a monitor, keyboard, mouse, speakers, controller etc.
For gaming you're looking at 2 factors, screen resolution and frame rate. Resolution is how many pixels are on the screen. You're going to be looking for 1080p, and 60fps. His friends will tell him that it's pointless going for anything below 4k and 120fps, but again, that's rubbish. It's barely noticeable and won't affect how he can play the game. So for the monitor, don't bother spending extra money on a 4k one, you don't need it.
For the PC itself, there's 4 components you need to focus on.
The CPU - I'm running an i5-8400k at the moment, and it's showing its age a bit. I'd be looking at an i5-10000 series for this.
RAM (Random access memory) - You want at least 16GB. 8GB will do in a pinch, and it's really easy to upgrade so you can always buy another 8GB stick later on.
Graphics card - This is the really important one. Unfortunately it's also the most expensive one. I'd look for a GTX 4060. Or you can go for one that's a bit older but more powerful, like the 3070.
Finally, the hard drive. Makes sure it's a Solid State Disk (SSD) rather than a regular hard drive.
If you're lucky, you may be able to find a PC with all that, and the monitor etc, for around £600, that'll play pretty much all current games at 1080p.
Good luck!