What are the the legal ramifications to an employee sending private business documents to her personal email address?

First, if that email address is shared by or accessible to others, it may remove or defeat the confidentiality of the documents (assuming they are confidential): if an employee shares confidential documents with other persons, that can render them non-confiidential.

Second, if it violates any technology policies, any confidentiality agreements, etc. signed by the employee, it can result in him or her being fired "for cause" (no elegibility for unemployment compensation), since it would be a violation of workplace policy and/or contractual obligations.

Third, it does NOT give the employee him/herself any rights in or to the documents--they remain the company's.

Fourth, if the employee later does or is alleged to have done someting else improper, like stealing company intellectual property, conspiring with others to defraud or damage the company, illegal competition or solicitation of clients after leaving employment and in violation of any confidentiality or non-solicitation agreements, this can be evidence of the employee's wrongdoing and/or state of mind.