which is a high end one
It must be really high end, or the plumber comes very expensive. Even the fancy thermostatically controlled units with super-deluxe heads (nine different spray patterns, etc.) are only about £250 new. If it's an exact replacement (no need to change the position of incoming pipes and cables) then it shouldn't take more than an hour or so to change.
If it's just the element burned out and it's a current model, I would have thought it possible to obtain a replacement element assembly, which would normally be no more than about £40, often less. It can be difficult to source spares for some models though.
Just out of curiosity, do you have the make and model number? (I know, probably not as it's a friend's house.)