The muzzle is a good idea in case an offleash dog runs up to her or you somehow drop the leash. You can get an inexpensive nylon one at most big pet stores. Also, a muzzle is a great red flag to other owners, who might otherwise come hopping up the sidewalk to say hi and chat–dog walking is a very social activity, after all.
The best thing you could get though is a head halter, a brand called the Gentle Leader. It works just like a head halter for a horse, lets you control the dog’s head (and thus where they are looking or able to reach with there teeth) and purportedly also applies pressure to an acupressure point which calms them. Properly (snugly)adjusted they are very comfortable (1 finger fits behind the back of the head, 1 finger fits at the muzzle) A lot of people try to make them looser because they think that would be more comfortable, but it just makes it slide around more and be more annoying, plus you or more likely to lose control of the dog’s head and it will slip off. The GL is about $21 but save a lot of aggravation, fear, and is a great training tool. Get a can of that cheese whizz stuff, and whenever you guys see a dog coming, take an avoidance route, talk to the dog to get as much of her attention as you can, and give her some cheese while praising her for not making a stink.
As long as you’re consistent, and try to never go over her threshold (that place where she feels stress/fear and begins reacting to other dogs) you will begin to be able to go closer to other dogs, because she will begine to have more positive associations: “Hey, that dog is fifty feet away, but I’m getting cheese and praise, who cares what that dog does…”
The first thing to do when she starts reacting (stiffening, barking, growling) is makes sure you break that eye contact. Just like people, eye contact is a potent emotional thing, and this is where the head hater really earns its keep. Walk behind a parked car, get her to focus on you, and give her a treat for that.
I am a big fan of the head halter, as you can tell. I think it has saved a lot of dogs, incorrigible pullers are more easily controlled and suddenly walking isn’t such an ordeal; but you may run into a few people who think it is a muzzle. It isn’t. As you will see, the dog can eat, drink, pant, and get their mouth open wide enough to grab another dog if you make a mistake, so please consider a muzzle as well if the dog is really aggressive and the dog population so dense you think you might find yourself surrounded ;).
With consistency, you can see a difference in just a week, but it takes consistent considerate positive reinforcement throughout the dog’s life to really effect that change in a more permanent sense. Your friends will thank you (or should anyway) and given the pain of having your arm yanked out of its socket and dogs and owners “freaking out” about the aggressive dog, a head halter pays you back in no time.
Initially, the dog may be annoyed by the halter, but give distracting treats and praise and eventually they’ll become used to it, and even enjoy wearing it like any other collar.
BTW, I am not affiliated with the halter in anyway, I just used to train dogs for a local humane society and think it (and consistent positive reinforcement training) really is a lifesaver for many dogs.
Good luck!