For anyone still following the 2009 iMac, found out why they system wouldn't boot up from the internal HDD. Booting into Verbose mode showed it getting stuck in a loop of "too many corpses being created" - that made me lol.
So with a clean High Sierra 10.13.6 running on the external HDD I was able to use Migration Assistant to transfer all of her User profiles/Apps/Pictures and other files onto the external so it now runs as it should without getting into this "too many corpses being created" loop
I did find people suggesting things you can type into Terminal but it didn't seem to be a 100% fix, it worked for some and not others. The only real way people found was to restore from a TM back up or a full wipe and reinstall of macOS.
Anyway, with the copy on the external drive I can now open it and install the SSD and then go through the whole Migration Assistant process again and hopefully that'll be it done
So with a clean High Sierra 10.13.6 running on the external HDD I was able to use Migration Assistant to transfer all of her User profiles/Apps/Pictures and other files onto the external so it now runs as it should without getting into this "too many corpses being created" loop
I did find people suggesting things you can type into Terminal but it didn't seem to be a 100% fix, it worked for some and not others. The only real way people found was to restore from a TM back up or a full wipe and reinstall of macOS.
Anyway, with the copy on the external drive I can now open it and install the SSD and then go through the whole Migration Assistant process again and hopefully that'll be it done