Let's see, ways to disable a ford.
-Unplug brake switch, so car will start, but won't come out of park
-Swap out ignition lock cylinder for a different pattern
-Remove ECM fuse
-Trip the inertia switch, as has been said already
-Unplug coil pack, if it has one
Why are we disabling her car by the way? DWI or something? What about taking the plates and registration off the car? Would that work? I think probably the best way, if you need the car completely disabled, would be to just remove the ECM fuse and take it, along with all the spare fuses with you. She's not likely to know that its missing, and even if she does find out, she won't know what fuse to get to replace it. Or better yet, put an already blown fuse in its place, that way even if she decides to look under there, she won't see anything missing.