Hello, thank you for using this site to help you with your problem. My name is XXXXX XXXXX I have been in the dog field for 25 years. It will be my pleasure to help you today. Please understand that I may send an information request so that I may gather what I need to better help you with your problem.
Just to be straight, your dog is Sadie and is 15 , what is the age of the other dog?
Is the new do female as well?
It seems you have left your computer. IF you can tell me also is Sadie possessive over items like food, bones, toys etc?
Please give an example of what might start a fight.
Since you are off line now when you have time if you could answer the questions I will be happy to help you
the other dog will move to Sadie and Sadie will show aggression
Thank you. There may just be an adjustment period needed or Sadie could be worried she will get hurt or Sadie is possessive over things in the home. It is difficult to say with so little information what exactly the problem is. What I would try is first to put all toys, bones, & food bowls away to make sure it is not possession. Then I would leash the new dog let your friend hold the leash and allow the dog to be in the same room with Sadie, but at the same time you will reward Sadie with a high value treat. such as hot dog or chicken when she behaves no growling or snarling. But you must show her that you have the other dog under control or else she will continue. We call this behavior adjustment training. The premise is to have Sadie understand that when she reacts appropriately something good happens as well this only happens when this new dog is near thereby eventually having Sadie not mind the new dog being near her. You will have to watch Sadie's level of distress when you bring the other dog around her and know when to move the new dog back a bit. Everything has to be done in small increments until you get to the day the two dogs can be together in one room without problems. IF you rush it then you will have the same problem and it may create more stress on the part of both dogs which can lead to more dislike between them.
I would definitely get a command such as leave it on both dogs so you can stop Sadie's actions with a command. I would use clicker training as it is positive reinforcement training. If you are not sure how to teach that I will direct you to a site that will take you step by step in basic obedience.
I will also direct you to a site for BAT training which has videos and will give you an idea on how to train a dog gradually and using reward.
Once you have rated my answer I would like to schedule a free follow up with you to see how things are going or if you need more help with this problem. Should any new problems arise, you can open a new question and request me if you wish for me to help you again.
Here are sites for clicker training that will show you how this is used.