Actual it seems like, he agreed with customer ops manager about conditions, but HR didn't assign him to customer ops, but to another engineering department.
So basically, such an important thing for him, he should have confirmed about the conditions with the new manager at least.
Once again highlighting the importance of getting anything important written down in a verified document. After a conversation with anyone about this sort of thing, send an email outlining your understanding to the decider and push for confirmation it's accurate.
They might still not honour it, but at least you'll have some sort of leg to stand on.
So basically, such an important thing for him, he should have confirmed about the conditions with the new manager at least.