All credit to the umpire messing around!!
What umpire should do is to overrule the linesman if it was obvious that the line judge has made a mistake (there is also a service judge on the court who can unofficially help the umpire with descret gesture (for example pointing finger down for in) showing his oppinion about in or out) ...
If not, he should go with the linesman, and insist to play on - if any of the players doesn't want to, he has a yellow and red card in his pocket and he should use them ...
So, maybe the referee should change the umpire

