CGT is assessed on the gain - the difference between what he sells it at now and what it was worth 10 years ago, but less selling expenses (estate agent, solicitor). Then he can deduct an annual allowance of about 310k from that figure, and he pays 18% tax on the result (unless he's a higher rate taxpayer).
There may be ways to minimise it (if a big figure) so probably worth talking to a tax consultant. For example, it is feasible to live in it for the next 6 months before selling?