How to get a parents parental rights terminated when she has no custody of her children.

The children's natural mother is a meth addict and her children were taken away by DFS 2 years ago. My boyfriend was awarded sole custody of the children and she was given visits if she could provide clean drug tests. It has been almost 6 months since court and she has not made any child support payments and has not made any attempt to see the children. We are getting married and I want to adopt the children. We were told that there may be a law where if she doesn't make any payments or visits for 6 months where we could file to have her parental rights terminated. Is that true?