Call the non retriever back to sit. Hold her there while the other dog does the retrieve. Do this with dummies until she does it without fail before trying it with real birds. Good, consistent training will cure a lot more bad habits than a Ecollar.
I never let any dog run off to retrieve on their own. They are released to do it on command every time. Learned that lesson while hunting with a buddy and his dog. Covey flush on the side of the road just before a feed truck came along. You guessed it. Dog went to retrieve a bird which was wing clipped and flew into brush across the road. Feed truck front and both rear wheels hit the dog. A nasty sight. After that I added training to all my dogs so they were to stay or sit until released by command to retrieve. Easy to do if you start it from a puppy.
