The situation of Nick and his mom sounds dreadful. If you do call her, I would concentrate on sympathizing with her and seeing if you can encourage her to improve their circumstances. I think if she feels threatened, which she is almost certain to do, she will most likely just cut ties with you and your D, as gouf78 suggests.
While it is not okay in any way for her to abuse him, I don’t think the situation is going to be improved by starting a process that could lead to her ending up in jail.
Is there some kind of support group for caregivers that you can talk to first, so that perhaps you can offer her some concrete help?