I had a case in our hall when I was an elder when a married woman admitted after a talk about honesty that she gave oral sex to many boys when she was growing up in a different hall.
Whilst we helped her and her husband (who naturally was miffed) through it we had the duty to notify the other congregation of the boys, teenagers, what had happened.
We sent a letter...heard nothing...no action was ever taken against them.
Tell your brother to deny it, even if she saved the picture just get your brother to say he lost his phone for a while so any messages he never sent.
There is no way in hell they would write to the branch over a minor issue like this.