Ok thanks folks. But I’m not sure is he going paying any tax in Germany.
Presumably he hasn't been, up to now. And this may have flown under the radar, so to speak, because he hasn't lodged any tax return in Germany inj relation to his rental income, because he didn't realise he was supposed to.
But now comes the point where this strategy may come unstuck. The German tax authorities may not know about rental agreements affecting a property unless and until someone tells them, but they do know about the sale of a property. Notifying the public authorities about a land transfer isn't optional; the only way you can transfer land is by registering the transfer. In Germany there's a property transfer tax (varies from 3.5% to 6.5% depending on what state you're in) so when the buyer of the property lodges the transfer with the land registration authority, the LRA tells the revenue authority so that the land transfer tax can be assessed and demanded. I don't
know that the revenue authority uses the notification also to check on potential CGT liablities but, given the German reputation for bureaucratic thoroughness, I think it's more likely than not that they do.
If there is going to be a CGT liablity in Germany, the OP's friend wants to know that sooner rather than later, partly to avoid interest and/or penalties, but also because any CGT payable in Germany is deductible from his Irish CGT bill — but only if it has actually been paid. So they want to sort this out
before the time comes to settle the Irish CGT liability.
If I were the OP's friend I'd take some advice locally in Germany about how to recognise and settle his German tax liablities before proceeding to sell the property. There may be things he can do to minimise the problem and, if so, he wants to know about them while he can still do them.