Getting the proper nutrients is always going to be a good thing, whether a person is on the spectrum or not. Everybody functions better when they are not also dealing with nutrient deficiencies or food intolerances. It is, however, troubling the number of parents who seem to think that autism will 'go away' if they can just get the right diet in place ... and how many go about attempting that without medical testing.
My son is such a picky eater that it would be impossible to get him on any kind of special diet. Anyway, I tend to think that unless someone has a specific, proven, allergy then a healthy balanced diet is good for everyone. Everything in moderation is my thinking! 
One of my problems is that there is a such a drive now against obesity in children that the message my son is getting at school is completely the opposite of what he should be getting. He is 6 years old, 3 feet 6in tall and weighs only 2 1/2 stone. Yet he comes back from school and refuses food because he is anxious about putting on weight! Grrrr...
I have spoken to the school nurse about this and she agrees it's crazy, but it's government policy across the board which seems to assume that all children are obese already or heading that way. Instead of telling children not to overeat, they should focus on healthy eating instead. Having said that he does get free fruit and milk, and currently they are piloting free school meals for all in this area.