Although the first answer may be to have your child stand up for themselves by engaging in a little conflict resolution, this may not address the problem.
On the other hand to have them role play and reverse their situations would be more appropriate.
Most parents are so sensitive about their children that it may lead to a bigger argument between you and the other parent for you to bring it up.
But for your child's sake it needs to be handled ASAP. So role play it is. Most children will respond to indirect confrontation if it is done in a loving manner.
Find out from our child what the last incident of bulling was. Then you can invite the other child to role play, reverse the script and when you see the other child getting uncomfortable pull the plug and present the teachable moment.
Point out that if the other child felt uncomfortable then they should imagine how they make others feel when they act in an overly aggressive manner.
Let them know that they are accepted but that no one likes a bully.
This same thing can be done by making a short story video that has bullying in it in then you can use the short story video as an excuse for the role playing.
If you can; get the other parent to participate with you in the making of the video.
Post it on you tube and you'll all get to enjoy it.
No doubt this is a delicate situation, but the wisdom and will to do the right thing should make it all work out for the better and keep the aggressive child from maturing into a adult menace.