I realize mom cannot do it but she can encourage those on good terms with the custodian to encourage the custodian to do these things. An older person caring for a toddler/preschooler would likely appreciate having the child in an outside program. The preschool will require (or provide) healthcare. If there is the possibility of poor care or abuse, having the child in a supervised situation is a non-confrontational way to insure the child's wellbeing. If mom regains custody - physical and/or legal , she should be sure to take advantage of available programs.
DD's current problems arise from her inability to take charge of her life in the past. At the time, she was apparently unable to negotiate the "system" and is now in a place where it is harder to undo what has been done than to have done it better to begin with. There are likely many good reasons she was in that position when the other grandparent took charge. Having health problems of any kind greatly impact a person's ability to handle new and stressful situations making it far easier to be manipulated by a "stronger" personality which sounds like the case here.
I am concerned if there are accusations of drug abuse that there may be unresolved legal issues especially if any of her exs or associates have been involved with illegal activities.
She can also prepare to be a great parent by taking advantage of the classes at local social service agencies; classes in strategies to cooperate with the other parent would also be of benefit. She should be prepared to counter claims that she cannot parent by showing that she is making an effort to develop these skills. I wish I had known when my children were little what I know now - and am still learning; I did not have access to these resources, and my parents drew up during the Depression and did not know what was becoming available to direct me when my kids were much smaller. Young people do not appreciate the resources they can now access, but they have to know where to find them.
IMO, DD need to get a job and file for both visitation and to pay child support to the custodian or if she is now in arrears from an existing order, to make an effort to clear these up.
If DD feels good about herself physically, mentally and emotionally then she will be empowered to deal with the legal issues.