Are you sure she is behind?  If her mother is not actually teaching her, you could notify the school district.  Homeschooling is legal in all 50 states.  Regulations for testing are different for each state.  Most states have standardized testing in grades 2,5,8 and 11.  You can purchase these standardized tests yourself and have her scored.  I have included a link to Seton.

I know that in order to homeschool your child there are certain regulations that must be followed, and are monitered by the school district. I would contact the local school district and see what can be done. If your child is not learning then she will have to either put her in public school or find an alternative teacher to come into the home.