Electric shower

You can't go wrong with a Triton XR 9.5kW. Triton's a good named brand with a guarantee.

Alternatively, you could opt for a New Team shower such as the Cascade Waltz Electric Shower, which is made by the Bristan brand. It's reasonably priced and does the job.

If your friend has a bit more money to burn, then I'd recommend something like an Aqualisa Quartz Electric Shower. The great thing about Aqualisa showers is that almost every part is interchangeable/ swappable as they have a complete spare parts catalogue. I know it doesn't sound good when you're just about to buy, but I've had to replace a whole shower unit just for the want of a single part on a cheaper shower model.

Best of luck!