You've taken the right approach- speaking with your child about the behaviors that are being displayed and how they affect others. Your child may like this friend and is copying/mimicking those behaviors which makes the child feel closer to, and more accepted by the friend/bully. Copying these sorts of behaviors also takes the attention from your child as your child redirects the attention onto another child who's being bullied.
If you're to speak to the child who's a bully, you may want to use as an example a recent situation that was witnessed by both of you (parents) Inquire about whether or not their child displays similar behaviors at home or if they've gotten any notices from school or other parents about the unhealthy behavior. If the parent you're speaking to says that they have not witnessed or heard anything of that sort, then you can point out some examples of what you personally had seen/heard during the children's play or interaction.
If the parent acknowledges the information you're sharing, then you may ask for their cooperation in addressing this issue with both children initially and then for that parent to ensure that they monitor their child's behavior and consequent negative actions. And, you would be doing the same with your child.
Asking for the other parent's help is less threatening than if you're to just confront them. It would make them less defensive and most likely willing to side with you as an adult and a parent in finding ways to stop the bullying.
10 Days to a Bully-Proof Child: The Proven Program to Build Confidence and Stop Bullies for Good