Neither can you always think clearly and respond calmly when faced with the imminent collapse of a case that you have spent years working on and thousands of dollars funding.
I can certainly see how someone would be upset and angry in such a situation and might lash out. A less understandably instinctive response in the circumstances, I would suggest, is to deliberately write up a lie about a picture of a young abuse victim, not unless dishonesty is one's natural inclination to start with.
This lie was clear, deliberate and is now conceded. It just happened to be one thing in his story that could be checked. What about things that can't be checked? Lies of this sort are not usually in want of company, so it is unlikely to be the only fabrication. That is not to say the whole thing is a fabrication, just that what can't be verified no longer deserves any benefit of the doubt.