The tax is usually paid when you transfer ownership of the property but if the contract is in bitcoins there is theoretically no “real” currency transferred. Legally it becomes a “trade” rather than a sale.
If the tax in that jurisdiction is based on the assessed “real currency” value of the property transferred (regardless if it is sale or trade), then he will have to pay the tax.
If the tax is based on the amount of “real currency” actually paid for the property, then he technically owes no tax. In such jurisdictions you will often see homes officially sold for $1. These are property transfers between family or private cash deals to avoid the tax.
The problem with making such large purchases “under the table” is that you can't prove someone was paid. If something goes wrong with the deal, you can't prove you actually gave someone cash or bitcoin in court. This is why such deals are most often between family or trusted friends.