There would seem to be income tax consequences, has he owned this for long? Is there still a mortgage outstanding with interest payable? He will be liable for income tax on the rent received less expenses including interest on a loan used to purchase the property. As you say this does not make much financial sense, he moves from one property to another and ends up with an income tax liablilty as a result.
There could also be CGT implications if the apartment qualifies as his PPR as he will be liable for CGT for the proportion of the time it is rented versus the time it was his PPR if the apartment is ever sold.