None of those are really helpful, as lots of people who may never intend to set foot in the US again will have those. All USC's are required to file taxes if they meet the threshold, no matter where they are in the world.
London isn't particularly strict on domicile which helps, but anything to prove she's intending to move back would be ideal - email exchange with realtors, letter from family there if you'll be staying with them, evidence of job research, etc.
Good luck.