I have a ten year old stepson, and he is a timid soul who seems to hang out with bullies as well... It doesn't always work out to talk to the other parent, because they don't see what you do, as you stated. However, I've found a great couple of things to do that seem to have been working. First, when there is a friend that my son just needs to hang out with, I make sure they have lots of time together with ME. Yes, I invite them to come with us to movies, to scouts, to church, to pizza to hang at the house and do things with our family. I believe that when they see how we treat our son, his sisters included, how we love respect him, they are less inclined to be a bully. I am a semi retired school teacher, and all the kids know me, and like me. I use that to my advantage because they don't want me to think badly of them, and all kids have it in them to feel that way about other adults. I always state the things the bully child does that is good, what type of behavior I like I encourage.
Before you know it, there is no more bullying, it dissappears. Kids need our intervention, but in their lives as a positive influence. Going up to the parents just creates a maelstrom of guilt and defensive behavior, or it could... best to be avoided. After a time I noticed my son didn't want to hang out with this boy anymore, he saw things he did to others that he finally saw as bullying. He was over it.
Next stage, I encouraged my son to find ways to help his former friend not be a bully, and when he led the rally, others in his grade followed and the bully had no one to boss around anymore because the little dudes got organized, as my son puts it.
Yesterday, this former friend, bully and now much humbler friend rode his bike to my house. I greeted him with the kindness we always had for him, asked about what he had been up to since we hadn't seen him for awhile. He was subdued, polite and humble, asked where Levi was. He was next door with his new friend group, and I encouraged him to go over and play with them all. He looked scared to death. I called the cellphone that he wears when he isn't in my house, and told him M was there. He was happy to include him and be the bigger person. I relayed that information to M and he stood at least 2 inches taller. I think we handled things well, it took some time and definitely patience.. but we live in small town too, and these kids grow up together, and graduate together. I hope the greater good was achieved. good luck with your situation.
C.