He's probably not got enough space. He's probably bored, they probably come at him from above and not on his level. And he's probably not neutered.
If she wants to put him to sleep or rehome him it sounds like she's after a quick fix and taming him would not be quick and would not be cheap. Also he could be a she. :?
My advice would be ask her to ring a rescue or take him to Pets at Home at the very least. So he can at least have a chance.