@Normansglasseye I'm so, so sorry to hear that your DD has been assaulted.
In my opinion it needs to be reported to the police, if your DD is ok with that.
The person's behaviour is completely unacceptable. It doesn't stop it being unaccetable if (and we don't know this for sure) the person has a disability making him unaware of that fact - because if he cannot be held responsible, then there MUST be someone with him at all times who is responsible for ensuring he doesn't assault people.
It is completely unacceptable for someone to assault another person.
A man did something vaguely similar to me in the street some months ago and I reported it to the police. They took it very seriously. Two police visited me at home, a male and a female police officer. They took my statement and were very thorough; I felt taken seriously and listened to.
Being on the street in my case, they checked CCTV but unfortunately it didn't show it, apparently the camera turns round every few minutes to look the other direction.
Some weeks later someone from the police phoned me to let me know what they had done and again, I felt taken seriously and that they had tried their best to identify the man.
I have such little experience of the police (London Met in my case) that I was surprised how good they were about my experience and that they realised how serious it is when a man assaults a woman. You read online so often that the police don't bother, but clearly sometimes they do.
I hope your DD gets the right support and understanding and isn't made to feel that it doesn't matter what happened to her. It does and it absolutely should NOT have happened.