Oh I see - he thought he could avoid capital gains tax (CGT). Sorry, but that just won't wash.
We can't answer the CGT question because we don't have enough information. If this rented house is not his 'principal private residence' (and it seems it isn't as he doesn't live there), he is liable for CGT assessment on the difference between what he bought it for and what he sells it for. The tax rate is 18% of the 'profit', however it isn't as bad as that because he can deduct costs of buying and selling (mainly solicitor and estate agent, if used) and any improvements he has made that he has proof of. Also in the year of the sale, each individual of 'allowed' around the 1st £10k of a capital gain in the tax year without paying the tax.
That's as much info as it is possible to provide for you.