There are typically no immediate or legal ramifications for a mother who is caught doing her son's homework for him. However, there may be consequences for the son, not only from the school, but also with what he is actually learning.
While it is not a crime for a mother to do her child's homework, there are likely problems down the road for her kids. One of a parent's jobs is to teach responsibility and integrity. When children are rescued from doing the homework assigned to them, they learn that they don't really have to work. Mom will always do it for them if they ask. Plus, they learn to take credit for others' work.
Surveys from the Department of Education collected over three decades found that helping with homework does not increase children's test scores or grades in most cases. This was the same for all socioeconomic backgrounds. Parents took over the jobs and didn't help their kids, whether purposefully doing the work or simply helping too much.
Another problem is that homework assignments help children work independently and increase self motivation. Long-term assignments and projects help children learn to plan and manage work. These skills are lost on children who are not made to do their own work.