Possible, but very difficult to execute.
Most airlines scan barcodes on boarding passes at the gate. There are however, some foreign airlines that use the older two part boarding passes that you can tear in two. That has its own issue, because your character would then have to have one of those types of boarding passes, probably from a different US carrier, in order to sneak on the plane.
I think just about every international flight I have been on required me to re-show my passport to the gate agent, because the airline doesn't want to get stuck with flying me home. I can't guarantee that every international flight I have been on was like that though, so you could conceivably get away with there not being a passport re-check at the gate.
So, in order for this scenario to work here is what would have to happen:
Option 1: Your character simply slips through with the crowd and walks down the jetway without the gate agent knowing. Very low probability of this, but there have been a couple examples of stow-aways in the news in the last year or so. Your character would also have to slip by the flight attendants at the door. Because many airline passengers seem to not know where their seat is, flight attendants on most international flights on wide-bodies will usually re-check the boarding pass at the door to make sure the passenger knows which aisle to go down, and whether they need to go towards the front or back of the plane (for planes that board at a mid-door).
Option 2: Your passenger happens to have one of the older rip-style boarding passes to CA, which this airline also happens to be using and the gate agent doesn't realize it is for the wrong flight.
Either way, your character would also need to be getting on a flight that isn't full and stand around somewhere out of the way on the plane until everyone boards so that she can jump in an empty seat. If she were to sit down right away, the person assigned to that seat might start making a scene, which would alert the crew that she is a stow-away.