Back in 2008/9 I have come across various 2003 servers with AD issues. I wonder whether the above environment also hosted Exchange at some point in time?
I expect a probable CAL issue with the Reddit post, going back to the AD trouble, but to be still on 2003... I would avoid, it is not about untangling a mess any more, I would scratch and tell the client straight up what my recommendations are, accept it or get someone else.