It really depends on what sort of device she's going to be listening to using these headphones. If it's a CD player or Vinyl then by all means, spend copious amounts of money on the headphones.
However, if it's mp3s/computer then there's fuck all point in spending more than $50 on headphones because realistically you're not going to notice a difference. Even at the highest encoding she's gonna have pretty average sound and serious audio headphones won't help.
For what it's worth I'd recommend cans (ear covering ones) because they generally sound nicer, do less damage to your ears and many people find them more comfortable. Sure they're bigger and can't be stored easily but the sound performance makes it worthwhile.
Another thing to consider is whether she has piercings. This may sound weird but I get incredibly sore ears wearing cans cause they put pressure on my piercings. Plus the hot, sweaty environment of the headphones makes my piercings itch... not so cool.
I really like these ones,
http://www.newegg.com/Product/Product.asp?Item=N82E16826106324I had the lower down model, my friend splashed out and got these and they're much nicer sounding.
Some good brands are Sennheiser, Sony, Panasonic, Jensen and Koss. My Jensen headphones are fucking killer. I also have Sennheiser in-ear buds and while they sound killer they're actually way too loud and kill my ear drums so I'm gonna investigate getting some decent cans.