I understand that, so what he needs to do is sit down and start from this year and work back. It will jog his memory. He should check out his bank statements, credit card statements for things he paid for. There are obvious one's too like the bins every year, or whatever else one pays for as a landlord in the UK.
And I can just hear the accountants on here groan at your last post. He can't go to them with nothing. They aren't miracle workers and can't just find the figures for him. So first step is pen and paper ! My accountant showed me a box of receipts a client had brought in, it was a great filing system by the client, all stuffed in a corn flake box ! As you do.