First off, I live in Canada. When my mom was killed, my grandparents and aunt/uncle fought for custody over me. My grandparents go full custody, but I wasn't allowed to move until I turned 18. I was required to stay in the city so my aunt and uncle could see me. When we wanted to go on family vacations, we had to go to court so we could leave the province. So yeah, you could go to court and see what happens.
If your willing, offer her a place to stay at your house. It's going to be extremely hard for her, living on her own and raising a baby on her own.
If she absolutely refuses to take money or supplies from you, then take the money you were going to give her, and put it in a savings account. When your grandchild is ready to go to university/college, he/she can use it for their education. Because your sons girlfriend will not be able to support herself, her child and save money for their education. Not right now at least.
If she's going to go to university, help her out as much as you can. Offering her a place to stay is great, because it will save her money. Offer to baby sit your grandchild when she has exams. It will help her study and pass and give you time with your grandbaby.
If she doesn't want to or you don't want her to live with you, then invite her over for dinner or out to dinner.
Remember, this is just as hard on her as it is on you. You lost you child, she lost the father of her child, and her child lost their father.