No, not all dogs are as good w/cats as yours are but that doesn't mean that you were at fault. HOWEVER, this "friend" was careless in her friendship w/you and negligent in not telling you that her dog could be vicious. I refuse to believe that she didn't know. Don't blame yourself, you have enough going through your mind right now. You've witnessed a horrible tragedy but you are not responsible. Lavish your other cat w/as much attention as you can possibly give him. You all need one another right now. When the time is right, another kitten will make a wonderful addition to your family and a great buddy for Max.