My DS, 7, is on the spectrum and so is his BFF. They love each other and have sworn to be BFFs for life. They are both in second grade, were born a week apart, and have the same interests.
They did not become BFFs until 1st grade even though they were in the same class in K. This year they are in different classrooms bc they get into too much mischief when together.
DS despite his quirks is quite popular as is his friend. I wish the two of them would include some of the other kids into their group and DS would accept other play date offers but no luck so far. They are fully mainstreamed.