Like many have said...have her take a class. That will determine whether or not she's really interested in riding. If she is, get her something like a used Ninja 500 ( if she's interested in the sporty side), or a Suzuki Savage 650 ( if she's cruiser bound). I'd skip the 250s. There are larger displacement bikes that weigh only negligibly more that will be satisfying for much longer.
"...you can't lose what you don't put in the middle... but you can't win much, either.