Validity of nikaah

This is a question about validity of Nikah for my sister. Since her marriage, their relationship was not good. Husband would not treat her well and also never gave due respect to any of our family member.
In the 1st 4 year of marriage, my sister lived with her husband for about 1 year. Numerous attempt was made during that time to bring the two closer. With everything failing, my sister finally left her husband and now lives with her 17 year old son alone for last 13 years. Is their Nikaah still valid? How could she get out of this Nikah.