How long have you been working on this? Positive reinforcement can be very effective, but you have to make sure you're consistent and patient.
I worry about people using prongs with a dog that tries to lunge forward to meet others because I've had so many clients accidentally create a fearful reaction by using prongs for this exact purpose. If you put a prong on and do not use it correctly for a situation like this, then what the dog learns is I lunge at another dog, I get hurt. They learn to equate another dog with pain and lunging because they're excited can turn into lunging to get the other dog to go away.
For situations, like the surprise dog, it is just fine to simply turn around and walk her away. She may not always be 100% while she's working on it, no dog is. What is important is that you manage her environment to set her up for success. And if you have generally gotten rid of the lunging behavior, but she still pulls and whines, heighten your criteria. Just as you may have turned her away for lunging, now turn her away for pulling or whining. When she gives you a calm reaction, or better focuses on you, THAT is when she is rewarded. Otherwise, she is removed and she does not get to greet or watch the other dog.