Emotionally upset😢😢😢 i know it has nothing to do with pregnancy

So i have a 6 year old boy who is the most nicest caring and loveable kid ive seen in a while. I dont understand why parents think its ok for their kids to be so agressive? Today we were at a birthday party and i witnessed something that i will probly never forget i saw my son getting hit in the face over and over by his suposibly friend they say they just playing but to me that other little boy was just way to agressive. His mother pays no attention to her kids and didnt even bother correcting his bad behavior. My son knows nothing about fighting or wrestling and I hav told my son that he needs to stand up for him self and tell him to stop but that kid just doesnt understand. Ive been sitting here cryn because ive been replaying the image of him hitting my son and me fieling like i couldnt get to him fast enough to end it. How can i get my son to understand that what that boy does is not okay. Is this considered bullying?

There is no talking to the mother so thats off the list ive been telling my son now for a year and he just keeps saying but he is my friend

I do fiel bad for the other lil kid and wonder what has drove him to be so mean