oh she'll have a fantastic time. I worked & lived in Manhattan for a year and I still get homesick for it, even though the winter is really tough & the summer horribly hot. But I shared with an American friend - who now has a 3 bed apartment, one room with my name on it! My DS stayed there on his gap year & had an amazing time, he tells me. I don't want to know ... 
I'd say have a look at Craigslist. It's the way most New Yorkers find apartments or room mates. It can be dodgy, but if she's sensible it would be OK. But she'll need to know which area: "New York" is the 5 boroughs, and not just Manhattan. Manhattan is the "New York" of films etc, but there are cheaper areas to live: bits of Brooklyn (which is lovely) and now Queens is quite trendy. Bits of Brooklyn are almost as expensive as most of Manhattan.
IME, in Manhattan, the studenty area around Columbia past the upper west side may be a good hunting ground, and Harlem is really nice. South Harlem & uptown from there on the east side is cheap, but lots of the projects and no subway (you're stuck with the 6 line down Lexington), but could have cheap possibilities.
Also try AirBnB for a week or so when she first gets there. Then ask colleagues where she's working. TSR might also have some tips.
And there's always the Y uptown, and a couple of cheap downtown hostels. If it's in the summer, then NYU might have dorm accommodation in the Village & towards the lower east side. Ditto Columbia on the upper west side.