What can I do about someone I know who is letting their child be subjected to different types of abuse?

A girl i know who at one time was a close friend is dating or married to someone who is abusive to her in front of the children. she has been in the hospital at least twice in the past year that i know of. She is also addicted to drugs, and what kinds i’m not sure of. I know that one is meth because of the rapid weight loss and increased age in her face within the time period of about 2 months.
The man which i do suspect her being married to, has been in the county jail several times, and state prision once that i know of for abuse, assualt, and several other charges.
Why in the world would anyone put their children in that situation? She was a good person at one point in time, now she isn’t stable at all and hasn’t been in a while, she is 28 and has been through at least 3 marriages since she was 20. is there anything i can do to help these kids?
The problem with calling social services is that she has moved again i don’t know where she lives now. i’m really worried about the kids, i don’t know where her daughter goes to school at and her son isn’t old enough to be in school yet. i don’t know how to reach the biological father either, but i’m surprised he hasn’t interviend yet either. she has done things to me since she got started on her pill popping bit that i have quit talking to her. so, as far as that goes, i’m not going to be able to get a phone number or anything from her. her family knows about it too, and they aren’t doing anything to help her. it seems as though those poor kids are in a lose lose situation.
I am currently trying to locate her address. her last known address i know is from 2 years ago and she has moved at least twice that i know of. i know the town we live in the same town, but i’m not sure where. rest assure i am working on it though.