Thanks for the additional information. It is helpful. The first thing that should be done is to rule out a medical cause for the sudden aggression. You can read about these here:
If there is no medical cause for the aggression, then it is strictly behavioral. Dogs are aggressive toward other dogs for a variety of reasons. It might be that they are fearful of other dogs and thus are aggressive before the other dog can be. In other cases, a dog is aggressive in order to dominate the other dogs and be the alpha member of the pack. Other causes could be that the dog feels they are te alpha member of the pack and as the alpha member they must protect the pack (you) from threats (other dogs). In this case, it could be either of the first two reasons since she just got him.
In addition, owners sometimes make the situation even worse by tensing up and worrying about what will happen. The dog senses the owner worry and feels that he is justified in his aggressive stance because you are obviously worried about the dog. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the other dog.
To correct this problem, your daughter will need to have him obedience trained. If she can, I would do group classes (with the muzzle) and let the trainer know of the problem the dog has. It might take a few months of basic training before he is ready for group class. . Before she can get into classes, I am including links to a couple of other sites that teach some good methods of training. Be sure and read both.
The following site is helpful. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if she trains at least 30 minutes a day (two 15 minute sessions). I would start making her dog work via the Nothing in life is free program (NILF). It is outlined below.
I think she will be pleasantly surprised at how well her dog does with training. Dogs like knowing what is expected of them and they love the little paper thin slices of hotdogs that I use for treats while training. Give this a try and see how it works for her.
It will be helpful if she can find someone with a dog to help her once she has the dog listening to commands consistently. What she will do is have her dog on the leash. She will have her helper off in the distance. Her helper will gradually move their dog a bit closer to her preferably walking past her position in the distance. As long as her dog ignores them, she can give the dog praise and a treat. The second she sees him fixate on the other dog or show any other sign of aggression (hair standing up, etc.) she has togive her dog a correction by giving a short tug and a firm low toned "NO". It shouldn't take the dog long to realize she will not tolerate the aggression and that if he ignores the other dog, he gets treats. Once this happens she can repeat the training moving the other dog closer until he is no longer trying to lunge at other dogs. She will need to practice this when she and her dog are walking as well.
. In addition, if the situation is not improving using the techniques I describe, you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.