I think that the best thing you can do for your grandbaby is to be as supportive to the mom as possible. It is good that she will be going to her moms....I am sure that she needs as much support as she can get until she is stable.
Something that you might want to do is offer to take the baby for the day one day a week. That way you will take some stress off of her as well as allow your grandbaby the opportunity to be around other famiy members who can expose him to a less hectic lifestyle. Your efforts should really be to alleviate as much stress on your son's girlfriend's life so that she can get better.
I know that your son will have the baby for the three days and that will be another good thing for the baby. It sounds as if the girlfriend returning home to her mom was somewhat a mutual decision and it is good that she will be with her mom who will be able to keep an eye on things and help out with the baby too.
You sound like a good grandmother and supporting your grandbaby's mom will in the end be the best thing you can do for your grandbaby.
Hoping that things will get better all round for everybody.
~ Goody