The word 'pretense' is problematic, meaning that grandma may think that there's hope for Mom. She can, if she believes that Mom is using, report her to CPS (tell them she wants the kids, so they don't go into foster care, the worst-case scenario IMO), and she can also file for a guardianship of the grandchildren with the local probate Court. Sounds like she needs to find a good attorney in her area, and give all the facts about Mom's behavior to him/her, so that proper decisions can be made. Good luck!
Under Washington law, a nonparental custody petition will only prevail where the non-parent, in this case grandma, can show that the mother is unfit, or that placement with the mother (if she is otherwise fit) would detrimentally affect the child's growth and development. In other words, the mother must be "unsuitable". Washington law defines unsuitable to mean a situation seriously affecting the welfare of the child, among other things.
Generally speaking, Washington nonparental custody petitions are filed when a child is not in the care of either parent, and where are is active and serious drug abuse or neglect of some sort. Residing with a married man doesn't fit that scenario, and neither does past drug use. Likewise, suitability is not judged based upon the financial where-with-all of the parent as compared to the nonparent, so grandma's large house does not provide an adequate basis for custody either.
A parent and a nonparent are not judged under the same standard in a nonparental custody petition. A parent who is a less desirable parent will prevail over a more desirable grandmother unless the situation is very dire.
Given this, I do not recommend contacting CPS based upon the facts I heard above, as I did not hear any allegation of active drug use, neglect or active physical abuse by someone in the household. Further, under Washington law, grandparents have zero rights to visitation, so maintaining good relations with their children is a good idea, especially where there are borderline care issues that necessitate an active grandparent to watch over the situation. Contacting CPS would surely cause a wedge between grandma and daughter, and might affect grandma's ability to watch over the grandchildren.
I would recommend talking with an experienced family law attorney as soon as possible - one who specializes in nonparental custody work. This way all the facts can be analyzed and the attorney can properly advise the grandma regarding whether CPS's involvement is warranted and/or whether a nonparental custody petition should be filed.
Good luck - Nancy Nellor Retsinas
Sign up to receive a 5-part series of useful information and advice about child custody law.