Q: Can a shower hose be taken on planes worldwide without hassle?
Since it is not possible to answer for every terminal entrance operator in the world, the best you'll get here is advice based on experience.
While a hand-held shower might seem unusual on it's own, it's probably not that unusual among thing people take on an airplane, all totally harmless.
Using US/CA/UK as the standard, a hand-held shower is not prohibited. But if the screener doesn't recognize it or you just picked the wrong lane, you might get some additional careening. If this happens, 99%, it'll just be an verification of what it is and a swab test.
To reduce chances of this happening, perhaps chose a plastic model which would have less substance on the belt scanner.
Also note, in the US/CA/UK, if you declare this a medical device, especially if your friend has a visible condition, you back them into a corner making it much more difficult for them to refuse it, but only do this as a last resort.
Q: Is there going to be difference to the security if it's already used? Or does it have to be packaged?
It's probably better if it's unpackaged since it would make it easier to swab if it gets to that.