Poor dog. I'm wondering, too, if she has adequate shelter outside. Maybe you could at least get a crate and bring her inside at night. Here's a thread on crate-training:
Note that you can't just throw a dog into a crate. It does involve a little bit of training, but sleeping inside would be a far better alternative than leaving her out in the cold.
To be honest, all I know about your girlfriend is that she neglects her dog. The problem is NOT that the poor dog has eaten a sock, but that your girlfriend seems to care so little about the dog's welfare. I hope for your sake that she has some redeeming qualities and that she'd take better care of a child should you two decide to become parents. You seem like a really caring person. Good luck to you.