I'm not sure the school have a right to refuse a child admission based on HIV status,I would advise a visit to a solicitor or citizens advice bureau. I would also tell her to get in touch directly with the Local Education Authority in her area to find out what options are available for her son's education.It may be that they insist he attend a special school for children with disabilities,even though HIV is not a disability.
I wish her luck as the LEA are not exactly easy to deal with in any area.