KLM is pretty good for earning points across the Atlantic. 3 return trips and she'll have enough points to make Silver where the earning level rises on points. 6 and she'll be Gold with better point earning and lounge access. 9 return trips brings Platinum with better points earning and other benefits.
6 return trips would earn her one free return flight or 3 one way upgrades. As Gold she'd need 3 return flights for a free one and as Platinum just under 3 .
They're also aligned with Northwest and Continetal in the States so internal flights will earn as well.
Not sure how this compares to the other Airlines but I'm sure someone will jump in.
Nigel