Unfortunately, although it is government policy to include special needs children in mainstream school where possible as your friend's son's experience demonstrates it doesn't always work. Since they haven't supplied a teaching assistant, I'm assuming he doesn't have a Statement of Special Educational Needs.
Under the law of the land children under the age of 16 must attend school and while I'm sure the Education Welfare Office is sympathetic they are obliged to enforce the law. If the child's GP were willing to sign him off sick it might get them some breathing space while they consider their options.
They should try to make contact with their local parent partnership group, they provide help & support for parents of children with special needs
http://www.parentpartnership.org.uk/Templates/ map.htm
Information for parents of children with special educational needs:
http://www.direct.gov.uk/en/Parents/Schoolslea rninganddevelopment/SpecialEducationalNeeds/DG _10016184
also try making contact with the National Autistic Society, they should be able to put you in contact with a local support group
http://www.nas.org.uk/
If your friend's son isn't coping with mainstream secondary, it may be worth exploring whether or not there are local special schools where he would fit in.
The mainstream school as it stands can avail of a number of services provided by the Authority - the Educational Psychology Service for one (parents can also contact the EP directly & ask for assessments), they should be able to assess him, there may also be an Autism Adviso