I am the photographer here, and to answer your question, I think there is a big difference in engaging in behavior at a private function such as a convention/meeting and engaging in it in a completely public venue such as the London Tubes. I don't wear diapers in public because I view it as a private act, so the circumstance you've asked about would never happen. She outs herself by sucking her thumb in public. All my iPhone did was capture the moment for posterity. I maintain that if she felt her thumbsucking was private and shameful, she wouldn't have been doing it in a tube. Frankly, I'd be more concerned about the sheer number of germs she is exposing herself to by sucking her thumb after passing through tube stations and potentially touching things. The London Underground is a vile petri dish of the very worst of human filth, simply due to the volume of people that pass through it daily. She would be well off to consider some hand sanitizer if she insists on sucking her thumb in the underground after potentially touching any surface in the entire tube system.
It sparked me to read up on the subject, and I learned stuff, so I think sNs would not be opposed if I said thank you.