Yes, once let the mortgage interest would be allowable against the rental income. Indeed, in some of those far off days it was allowable against his income tax liability anyway, a concession finally abolished in 2000 by Gor***** ***** who described it as a 'middle class perk.'
As far as Capital Gains Tax (CGT) is concerned he has no liability as this does not kick in until he sold the property. As he is non resident of over five years it would be limited to any gain from a 6 April 2015 valuation. There is an Annual Exempt Amount, currently 11K, to offset any gain realized or possible Lettings Relief (up to 40K) instead. This would also be adjusted proportionately to cover the total ownership period and the let period less 18 months (when he is deemed to be in residence even if this is not the case). CGT is rather a complex area of taxation. I am aware that Oz does not have CGT, the gain being assessed as income, I believe.
Thank you for your support.