Hello Peter, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
I see your point, but your client is operating a rental business with multiple properties. As some revenue expenditure prior to renting out is allowable against rental income I see no clear reason against post rental costs of a reasonable nature being allowable also. However, one point eluded me until further research revealed this gem from Colman & Co Ltd:
'Where a taxpayer has more than one UK property, all rental income and expenses are pooled to determine the net profit or loss for the tax year'
Thus the expenses to which you refer in your question would be allowable as they would be in the pool.
I do hope that you have found my reply of assistance.
Thank you for your support.