Assuming you mean Electric Shower (electrically heated) not Power Shower (hot water from cylinder, with an electric pump) then an old rewirable fusebox will usually not be adequate.
I assume it is a Wylex Standard in Brown or Cream. Many of these had a 60-amp main switch, and were not intended for fuses exceeding 30Amps. If you tell us more about it we will be able to identify if your customer's one is different.
I suggest a new CU with either a 45A RCBO, or else an 30mA RCD and a 45A MCB.
As this will be subject to Building Regulations inspection, it may save the cost of upgrading the rest of the CU and circuits at this time if you fit a separate one.
If fitting a new CU, my preference would always be to fit a good big one, with plenty of room for the remaining circuits in the house to be supplied from here when the old fusebox and circuits are eventually replaced (hopefully before long). It would also be an advantage to fit a 100A DP switch in the meter tails before the Henley so that there is a main switch to isolate the entire installation in the house.
Are you complying with part P?