Okay here it goes. I got a call from my friend yesterday and he told me he's completely devastated and does not know what to do. He owns a 1 year old female Miniature Pincher and she hates his girlfriend. The problem is that they bought the dog together and they live together. It's not like she's a new person in the house, she's been there since the dog first came to the house.
She would growl and show teeth every time she comes to where she's sleeping. There's no way she can pick her up, because the dog snaps at her!! If she's eating (the girlfriend), then the dog would act as if they're best friends, wagging her tail and being all friendly..... basically it's the dog who sets the rules. She's absolutely fine with my friend and lets him do anything he wants!
Now the frustrating part is that the girlfriend really loves the dog and feels very, very sad. She was never harsh with her, always loving and caring toward her, so nobody understands what's going on.
Now, I told them that from now on she has to become the person that takes care of her primarily. She has to give her food, take her out and the dog has to learn that she need to earn every little thing...... before giving her food, she has to either sit down or something else, just to get the idea that nothing comes for free and that the girlfriend is the one that MAKES THE RULES!
It might be funny and cute for some people to see a small dog growling and showing teeth, but it's absolutely unacceptable!!! Imagine you had a Dogo Argentino.... it wouldn't be quite so funny anymore! A dog is a dog and the same rules apply for everyone....... I can't imagine my Chihuahua showing any aggressiveness toward anyone in the house. Or toward strangers for that matter. Since day one I was teaching him that that's not something we will tolerate!

Now, I just wanted to see whether you have some ideas how to address this problem....

Sounds like a good idea.

I think a light lead on the dog at al times so that she can safely insist the dog goes where she tells her to, is a good. as it will stop the dog being able to get into positions where it can threaten.

How about looking for some information on calming signals for dogs. Maybe the woman's body language is saying different things to the dog. You know how some people can just make every dog wag its tail furiously (my dad is one). Turid Rugaas is supposed to be good and has a book and DVD called Calming Signals. There's some information on this here. Learning to speak a bit of dog might help.

I think the girlfriend should buy a book on clicker training and get clicker training the dog.  Especially if the dog is her best friend around food....

Thanks all...... I'll print it all out and give all the info to my friend. I think that clicker training idea is actually very good. Obviously food works with her, so that will be very effective.