Hi Neadtwailn Ncaazr,
If he did not declare that he was renting out his home while overseas on his P85, he will need to register for Self Assessment, as he will be a non resident landlord.
This can be done here:
Check how to register for Self Assessment
As he is a UK national, he is entitled to claim for personal allowance, no matter where in the world he lives (this is done on SA109).
As a non resident, he cannot complete the online HMRC taxc return.
He will need to either submit a paper SA100, SA105 and SA109, which can be obtained from the Self Assessment orderline here:
Self Assessment: forms ordering
By 31 October each year or submit an online commercial tax return, which can be purchased for a fee here:
Self Assessment commercial software suppliers
A government gateway user ID and password is required for this option.
He can do this here:
Personal tax account: sign in or set up
Thank you.