Have your friend encountered any crime so far? I was told that the crime rate in Manchester is really high.
I'm a Manchester student too, and whilst crime does occur, I've never personally felt threatened (and nor, to my knowledge, has the friend I mentioned earlier). I don't believe that it's any worse than any other big city, and there's a mantra that should be followed: don't be an idiot. Don't walk through the park at night, alone, flashing your iPhone whilst drunk, for instance. If you do, you may very well run into trouble. But actually, if you follow really basic personal safety advice then you should be absolutely fine.