How sure? Perhaps she has a scat fetish that her boyfriend doesn't share. Yes it's subjective but not necessarily a chosen response; most people find the image of a mutilated corpse or a festering wound to be disgusting for example.
'One of the prudish detectives wasn’t a fan of the photos, calling them “disgusting.”'
That's all we have to go on. A single word remark reported by the plaintiff. It's pretty much impossible to make any bearing as to the professionalism or otherwise of the "detective" (it says earlier in the same para that it was "officers" operating under a warrant, which would almost certainly not be detectives for recovering a stolen laptop, YMMV the piece looks unfinished and unedited to me).
However, FWIW, saying something is disgusting isn't casting a moral judgement on it. If you show me a festering sore I might remark "urgh, that's disgusting" but I don't find sores to be immoral. I'm pretty sure he or she wasn't presenting or even attempting to present a legal judgement with what was most likely an off-the-cuff remark. Unprofessional, perhaps mildly.
For all we know the remark was disgust at a teacher having apparently knowingly handled stolen goods taken from another school and encouraged a pupil into delinquency by helping them profit from crime and that the plaintiff misinterpreted. Yes it would probably still be mildly unprofessional to comment.
Aside, aren't school laptops marked to show their owner; this would have made it near impossible to not know that the goods were stolen.