[sweetlee]

If your dog is "backtalking" your girlfriend, he/she has no respect for her. If she is around the dog alot, or lives in the same household. She needs to work with the dog more.
She needs to not ignore it either, for yelping or snapping can escalate into much worse, such as biting. Is the girlfriend providing rewards for followed commands? A dog who knows it is going to get treated for good behavior will show good behavior.
I would have her provide care for the dog for a while. All things good need to come from her.
She should take over the walking, feeding, playing. And no food until a command is listened to and obeyed without challenge. Do a search on NILIF, it's not a short program, it should be a lifestyle. Having said that it is not the answer for all dog problems.
If your dog is not in training currently, I suggest you get it enrolled in either a basic obedience, or if you have done that already, an advanced class. Look for a trainer who uses positive motivation/rewards, but also one who will show you how to correct bad behavior. My trainer says we will never Punish our dogs, but we will help them understand what it is we want of them. Have your girlfriend go along and handle the dog at classes. Your dog is at the age to start testing, Liberty is looking to up his/her place in the pack. Someone said whoever controls the foodsource controls the dog. Not sure if this is totally true, but I do believe it's a good start. The dog needs to sit and wait while she puts the food down, and not go to it until an OK is given. I would start by having the girlfriend hand feed the dog it's entire meal. Making Liberty sit, or down or shake or whatever before each handful. Rotties will work for food. Also, no pushing past at doorways or gates, people go first and dogs are then invited.
If your dog is sleeping in the bed with you, stop that right now, and take away furniture priveliges if they have been given. Not sure how you treat the dog, and I treat mine like a person sometimes, I think we all do, but Liberty doesn't deserve this from you or the girlfriend until some manners are learned.
good luck,

Sharon