Same Apple ID uses different passwords on different machines

I’m trying to help a friend who’s not good with tech. He has an iPhone 12 and a Dell laptop. He’s been having trouble accessing his iCloud email via Windows Mail on his PC. I reset the app specific password and he can now receive iCloud email but not send. Further investigation revealed an anomaly with his Apple ID.

He has an Apple ID using his iCloud email address but he logs on with different passwords depending on whether he’s logging on from his PC or iPhone. I’ve checked thoroughly and the Apple ID is definitely the same on both devices. First of all I didn’t think this was possible and second when I tried to change his password on his PC to the same password as is on his iPhone it told me I couldn’t use a password I’d used previously. I would try to delete and reset both passwords but I’m slightly concerned that we have two different accounts here (despite the same Apple ID) and I might end up deleting one entire account.

As far as I can tell the data (received emails and files etc.) is the same when viewed from either the PC or the iPhone, however I didn’t do an exhaustive check so some might be different.

In all cases I access via “appleid.apple.com” then “manage Apple ID”.

I would be grateful for any advice as to how to resolve this situation.