The montessori school that my DD went to and my DS now attends had an autistic child there at the same time as my DD went. The nursery staff (the two main teachers actually own the nursery) were really good with this child. They set the boundaries for all the children and all the children SN and NT all fitted in with those boundaries. He was not treated like a special case which I think has served him well as he is now settled well at a mainstream primary school.
What really strikes me as different is the way that the other children and adults, just accepted the way he was. Some of the SN threads on mumsnet upset me because the SN children are not treated the same as their NT peers - e.g. party invites, invites home for tea etc. That was not at all my experience with this child. He was invited to all the birthday parties and his mother chose whether to attend on the day, depending on his mood, which all of us understood and didn't feel let down on the day if they didn't turn up. He had two birthday parties whilst he was at the nursury and they both had full turnouts by the other children. He was part of the tea after school thing - he came to our house at least as often as any of the other children and we went to his in the same way. What facilitated this was, I believe, the way the nursery treated him - i.e. they treated him the same as everyone else so the adults and the other children did too.
He also had a part in the nursery nativity play - no silly exclusions here either - whether by design or not he was given a non-speaking role which did not place him under too much strain (co-incidently or not my very speech delayed DS was also given a non-speaking role which again helped him enjoy his first nativity without being under pressure to speak!).
Whether this was the montessori way or just two sensible teachers I don't know.
In terms of the montessori teaching principles there are two that were particularly helpful to this child.
1 - the children are taught that things need to be put back exactly as they were, on the same shelf in the same position and in exactly the way they were at the start of play. So if you take the flour and lentil mix down to sieve it, it goes back in the same place and if you've spilt some you tidy it up and replace it from the pots set out on the shelf. I'm not sure if your DS is the same but this child used to melt down if things were not as he expected and this really really helped him.
- The children set their 'work' out on mats on the table. If another child comes along and wants to join in their activity the child is given the choice as to whether they want someone to join in. No pressure is put on the child to 'share' in fact if anything sharing is somewhat discouraged - seen as a bit rude to interrupt someone else's work!!!! This again, was enormously beneficial to the autistic child - he didn't have to cope with the social angst and uncertainty of having another child thrust upon him which may at some points have been a trigger for meltdown.
I'm so sorry to have made this so long but I do hope it helps!