When you see another dog coming, make yours lie down and wait till they have passed. Try to do this as far away from the other dog as possible to start with, then gradually wait a little longer before dropping her. You can put your foot on the lead to give a bit more security to keeping her in the down. If necessary bend over her and push gently on her shoulders to reinforce the down position. Use treats to reward if she makes no attempt to get up. If this starts to work then let her sit as they go by, and again ensure she stays and reward with treats. You can give her the treats as they are passing to distract - anything that keeps her attention on you.