EMA doesn't affect any money you earn, it's designed to support you in your learning programme. Plus it's not based on money you earn - your allowance is assessed from the income of the adults who have responsibility for you in your home - whether that's your parents, another family member or legal guardian.
Child support is claimable as long as the child is in full time education (i think...?). Also, your friend can have a maximum of one day off sick per week and still claim EMA (again I think..!)
Personally I think EMA is rubbish, I can't get it because my parents annual income is over �30,000, but I get no help with materials needed for my course.. Last year it cost me approximately �300 in materials and that was only my first year...