thanks for your response
when you refer to 'cold boot' do you mean similar to windows/Linux 'live cd' as in running the OS from USB into RAM rather than install as a test then confirm its operational then go ahead with the install?
EDIT:
I am personally totally new too mac installation procedures TBH so would need clear instructions
thanks
Since you’re new at this, you would best tell us the exact model. Click on the Apple in the top left corner and go to "About This Mac". And then click on the button that says "More Info" or "System Report". It will show you the specific model number, something like "MacBookPro5,5".
High Sierra has more up to date features and software than Sierra, but is also less compatible with old legacy software. It depends on what software you are running. 10.13 High Sierra also has a h.264 video bug that only occurs under very specific circumstances. Most people will never or only very rarely see that video bug. That bug is completely absent in 10.12 Sierra, but I've only seen that bug in the wild in real world usage twice in 9 months using High Sierra on two such old machines, and in both instances it wasn't a big deal. Plus that bug is present in a couple of fully supported Macs too, so it's not the fault of the unsupported install.
Overall, I think 10.13 High Sierra is the superior upgrade on these old Macs, as long as they have SSD and sufficient RAM, which is bare minimum 4 GB, but preferably 6 or 8 GB. These requirements are the same for Sierra and High Sierra IMO.
Assuming it is indeed a MacBookPro5,x model, the process is as follows:
0) BACK UP YOUR DATA.
1) Download dosdude1's High Sierra patcher.
2) Follow the instructions to create a USB drive with the patched macOS installer on it.
3) Turn off the machine.
4) Plug in the USB drive and turn on the computer to do a cold boot of the Mac, while holding down the OPTION key.
5) If the USB drive can cold boot, you should see an orange external drive listed. Select that one and boot off it.
6) Do the macOS install.
7) Reboot to the USB drive and run the patches.
8) Reboot the internal drive and enjoy High Sierra!
I suggest that your uncle stick to what he has installed now, or install "Low" Sierra and be done with it (whichever one of the two applies).
There is no inherent advantage in having "the most recently-released" version of the OS.
What's better is to have an OS install that works well for you...
If I understand the original post, "Low" Sierra is not officially supported on that machine either. So, either Sierra or High Sierra will be unsupported installs. If you're going to install an unsupported OS, you may as well choose High Sierra, unless you have old software that won't run on it.
Why High Sierra?
1) Better compatibility with APFS. I wouldn't recommend an APFS boot disk, but there are still external drives.
2) Better more up to date native software, like Photos. Photos is hugely improved in High Sierra.
3) HEIF image support. This is key, since iOS devices take a lot HEIF/HEIC images these days. The support is only with certain software, but at least it works. These files are completely unreadable in Sierra.
4) HEVC video support. This is less important since these old Macs don't have enough horsepower to play most HEVC, but at least the compatibility is there.
5) Longer security support. Apple typically issues security updates for older versions of macOS, but usually the newer the version of macOS, the more recent the security updates.
6) Longer Safari support. Apple typically issues Safari updates for older versions of macOS, but usually the newer the version of macOS, the more recent the Safari support.
I would expect that High Sierra will continue to get some maintenance updates until 2020, and if so, these machines will be viable until about 2022. For Sierra, subtract one year from those dates.