If she goes down the replacement path can you get 'powerful electric
showers
or are the dependant on your incoming water pressure?

The flow is usually determined by the amount of water that can be heated,
not by how much water can get through the thing. Unfortunately, at under
10kW, you aren't going to heat much water and an electric shower is not
going to be good. The limit is around 4 litres per minute, which is less
than half the paltry amount the water supplier to require to provide and
about a tenth of what they typically provide.

The best fix is to throw it away and connect to your house's hot water
supply, which can usually supply far more hot water, although if the thing
has furred up, replacement with a similar device (or descaling the old one)
will restore the previous performance.

Christian.