First of all, I want to say it's NOT OK that your poor DD was grabbed at like this, OP, and I'm extremely sorry it happened. It's not ok. Her employment seems to have very poor practice at following this up. I'm not sure what any people-facing employer can do to mitigate unintentional harassment like this, since it's hard to predict, but some training would probably help. It won't stop people with severe LDs/Autism behaving in the ways they can often behave though, and I hope people realise that this is one of the many unpredictable things about living in a community alongside very different people, as we are all free to move around in our society. But it was a horrible thing to happen, and at her young age. I had the misfortune to experience similar assault aimed at my private body parts, as a student, and I'd be lying if I said that after the initial shock/outrage, it didn't have a more lasting effect. I think young women can feel more confident and safer if they learn some fending off techniques via a martial art or self defence lessons. This might help your daughter, going forwards? And I really do hope she's recovering from the shock, and that her employers step up to support her. 
If this sad thread has done anything, I just hope it's opened some people's eyes to the horrific position we unpaid parent carers find ourselves in while trying to live with and care for someone with significant additional needs. I have a son who is mid-teens, severely affected by his autism and significant LDs. He does grab at other people sometimes - luckily not at private areas (unless it's me, he pats my breasts and it is a CONSTANT battle to get him to stop. As in, he'll lunge for me, I bat him off, firmly say NO, PRIVATE, and in 30 seconds he'll try it again. Do please, all you perfect parents out there, suggest what I ought to be doing? Maybe wear a metal chest plate or something else?). However it is irritating and often a shock if you're patted at or grabbed by someone unexpectedly.
Fortunately, on the rare times he's done this with strangers, it's been immediately apparent that he has significant SEN and people have been extremely understanding, sensibly proportionate, compassionate and generous. Thank goodness.
Which is fortunate for him, and for us. If we ever are unfortunate enough for him to grab, entirely without malice or intent at sexual harassment, at someone who DOES decide to take it further, we are screwed. If an unsympathetic police officer got involved or someone decided to be really bloody-minded about it (because cognitively he is about 3) then potentially he could end up on some sort of register. It keeps me awake at night. If that happens, he's unlikely ever to be allowed to live in most residential care homes, which god knows are like gold dust anyway, and far from ideal in many cases.
Please, all you posters who are SO quick to say that an assault is an assault, or that intention doesn't matter, or even that if a young person is liable to grab at strangers then he shouldn't be allowed out, please can you try putting yourselves into his, and our position? We are desperate. Occasionally even suicidal. I have the responsibility for a child who will NEVER grow up, cognitively. But he will eventually become adult-sized. His hormones are kicking in. Living with him is a challenge every hour he is awake. We take him out of the house to stay sane and because he loves to go out and explore. We and his very good special school work with him, very hard, with all the tools we have at our disposal, to get him to try and understand that private areas mean NO TOUCHING.
In a few short years, his time at school will be over, when he hits 19. If we're very lucky, we might find him a residential place at a 'college', perhaps a SEN farm where he can do activities he enjoys. He will never have a job. He will never have a girlfriend or sex with anyone else. He will never be able to live alone, or go out anywhere alone. In fact, it may be deemed necessary for him to have TWO adult carers with him when he goes out as an adult. Where on earth, in these cash-strapped days, do you think the LA are going to find funding for that? Meanwhile, we, his parents, are in our 50's, ageing ahead of our years, bits falling off etc. I honestly think that caring for him will put us into an early grave. Both of us have been suicidal at times. There is no fucking respite care, we have tried to get a social worker allocated (can't even try to apply for respite without one) and are still trying years later. There are none, we are not 'desperate' enough. There are no family members who are able or willing to help and we have no friends who are willing to take him on during the day time. My older NT son babysits once he's in bed so we do occasionally go out. That will stop when he goes to university. It feels like a prison door is slowly closing on us.
So please, everyone who is so quick to condemn, think what you're saying. Imagine what it's like to be us. It's shit. Imagine if you can (it's very very hard) what it's like to be my son, full of curiosity and joy, but unable to process his environment, stay safe on his own or assimilate the modes of social behaviour that most NT 4 years olds are capable of.