I'd exercise a modicum of caution.
Its true that benzoylecgonine is a product of human carboxylesterases and is excreted via urine, but ester hydrolysis happens spontaneously all the time, its just EXTREMELY slow in dry powders. In the presence of an alkaline aqueous solution, ester compounds hydrolyze like no ones business.
If we were talking about a CYP-mediated metabolite (N-demethylation for instance) or a conjugate (such as a glucuronide or sulfate formation), then I'd 100% agree, those metabolites will not form spontaneously under virtually any conditions (unless those conditions are inside a chemistry lab), but claiming that its impossible for benzoylecgonine to form anywhere outside the body is a tough claim to justify.
Don't get me wrong, I don't buy his story. While I'd happily argue that its possible that cocaine could spontaneously hydrolyze to form benzoylecgonine, I can't believe that a cop touching her hair with a cocaine contaminated glove could result in its formation. Was her hair soaked in lye at the time? We know from some of the initial paperwork that there were complaints that the kids were dirty and unwashed, so I couldn't even suggest that there was shampoo in her hair that could be alkaline.
Nick is the kind of retard to take a fragmentary incorrect detail and blow it up to unreasonable proportions, so I would advocate being precise with language. He's a lawyer by training, if not by active practice. Careless claims are fodder for him.