I don't really know what the trainer was told, it's my suspicion that they weren't told everything because otherwise, as others have said, a responsible trainer (and this one is young but highly qualified) wouldn't have had the dog off the leash and unmuzzled in a public place where lots of dogs are exercised. It's not knowing all the facts that is one of the things that has made me reluctant to report. Once I report what happens is out of my control: the trainer could be prosecuted and make the local papers for it which would ruin a career they've trained for years for, and if I don't know for certain how much they were at fault I'm reluctant to do that.
It really doesn't matter even if the trainer was told that the dog had no problems, was well behaved, etc. Professionals make assessments based on their own observations. Not on what some customer tells them about their precious doggo. The trainer is absolutely responsible for this and if I were the dogs owners, I would be livid that the dog wasn't protected from acting in the only way it knew how. Yeah, the dog isn't trained yet. Trainers exist to make dogs safe for people and people safe around dogs.
Why does everyone want to protect the trainer? Training a dog in a public space with no leash? That's insane. No professional would do that. Or at least they shouldn't. It's so irresponsible.
I'm not an expert: I think the trainer was wrong and made a mistake but I don't know everything that led up to that mistake so it's hard for me to judge that they should be put in the position of potentially losing their career over it.
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You're not in that position. Report the facts to the authorities (including wherever this trainer got their credentials) and let the authorities decide.