Personally I don't let them meet other dogs while they are on leash,friends or not. They actually have to learn to ignore them and earn their free playtime by sitting and waiting for the release command. So that probably takes some distance to work at that.
Here's what you can do: increase the distance, turn away and reward her with treats when she is able to focus on you. If she isn't, you are too close to the other dog. Group classes will help you like Blanketback said already. Take tug toys with you so you don't always have to feed her when she sees dogs. Teach her "Leave It!"
Deja, GSD, sable WL, female, born 11 / 2013
Cam, Rough Collie, sable, male, born 8/2015