As widely agreed, changing the hard-disk for an SSD is the biggest improvement per pound. A replacement SSD from any reputable brand (Samsung, Intel, Crucial, Kingston, Sandisk) will have a drive-imaging program included, either by having it on the SSD (ready to put on a bootable USB-key) or as a download from the manufacturer's website. One uses this program and an external disk-caddy to make an image of the 'old' hard-disk on the 'new' SSD so that it can be directly swapped. When the new SSD is installed, use the increase in speed to make a Windows re-install (from Dells hidden partition and restore tools) much quicker than doing the same thing on the hard-disk before the copying.
If the user doesn't specifically need a version of Windows, then just take out the hard-disk, insert the SSD, and install Ubuntu linux on it. If you find it doesn't do what he needs for school/college, then do the disk-image as described above and continue with Windows.
Note that if the machine has Windows 7 it is probably too old to use, though it could possibly be 'upgraded' (newer security level at least) to Windows-10 depending on how old it really is. Forget about Windows 11.
Most machines will be happier with 8GB of ram than with 4GB. You will have to check if the ram is in a socket (or two) or soldered on the board. If there is socketed ram, it may be that all the sockets are filled (meaning you would need to replace all the memory-chips to increase the total) rather than having one socket empty which you could then fill with the appropriate amount of ram -- typically doubling the amount from 4 to 8GB.
Once the machine is sorted out, get the youngster a small third-party Arduino kit (under thirty quid) for him to practice programming useful stuff.
