This is an often-asked inquiry. You may want to give specifics for how long, time frame, what's her budget, what she likes/dislikes, wants to try/wants to stay away from, and if she's laid over to the point where she has to stay at a hotel, where? This might be key as her layover location might dictate transportation and location.
Also, is she independent enough to try public transit, or would she prefer taxis?
As Bulavinaka states, we've had a lot of discussions on this topic:
From two weeks ago - 4 hour layover on a Thursday afternoon/evening
5 hour layover at LAX - February 2011-
6 hours in L.A. - August 2010 - 70 replies
Lunch near LAX on Saturday - July 2010
Looking for good dinner near LAX - not impossible I hope! - June 2010 - 56 replies