He would now like to sell the house with sitting tenant

I bet he would! One of the bigger issues for landlords is an exit route.
It's often hard to sell with a tenant in situ, as tenants understandably
are not keen to allow viewings, may not keep the place pristine, etc.

on the other hand, selling without tenants in place means losing several
months' rent, paying rates, heating an empty house, risking
vandals/squatters.