ACHI is a subset of the RAID command and you can leave the bios on RAID but just not have an actual raid array of SSDs. With only one SSD it will pass TRIM commands just the same as having the bios set to AHCI.
I connected all the drives, the SSD and both 500gbs and changed BIOS to AHCI.
When I installed Windows on the SSD both 500gbs remained as a 1TB as if they were still RAID, but obviously RAID wasn't on, as I switched to AHCI.
So it's all good.
He now has an SSD as C: and a 1TB D:, plus, he's over the moon with the speed of installations, loading times etc.
I don't know how it worked the way it did, but it did.
Thx again for the reply but obviously I didn't get the reply before I started the installation which by the way I would have done your way had I read it beforehand although you did say it would be the same as having BIOS set to AHCI, so thx again.