My five year old is being bullied. What can i do?

I'm on mobile so I apologize in advance for formatting issues.My son is 5 and while he is very sweet, he can also be incredibly sensitive. It's how he's always been. He has gotten into his fair share of trouble, as most five year olds do.We recently got new neighbors (we live in four apartment buildings that connect) and they have a little girl who is 3. Things were very smooth and they loved playing with each other. I've even kept her for a few hours so her mom could run errands.I've noticed that we parent differently and that is okay with me. To each their own. In our family we don't spank and we try our best not to yell (it happens). In her family they do spank and they do yell (I've heard them through the walls). They've even warned me about it. Again, didnt bother me. I can't judge them as i know they are very stressed. Money is very tight for them and they are trying to do the best they can.A few weeks after they moved in, it started slowly but ive noticed that she singles my son out every time. It doesnt matter what he's doing. He could be doing something on his own and i will hear: "Ops son, stop it! You can't do that!" Or "you're making me mad! I'm the boss!" And every time he will start crying (he's sensitive and dislikes any confrontation) and he will run away.She usually left him alone after that until recently. He asked me what to do. I told him that he could tell her to:Leave me alone. Ignore her. Come get me. Try to work through it. Go inside the house and do something else.He's really tried to do all of this. Im always outside with him.Its escalated to her following him, pushing, hitting. Screaming at him for no reason. To the point that i will take her by the hand and walk her to her mom (who's inside.) I've put her in time out, I've gotten on her level but i don't spank her. I dont like disciplining someone else's kids that way, even though they have given me permission.The other renters all have children and we have never had any issues. They all play together, with the occasional fight over toys. The other kids have even told her: you need to be nice or you can't play.What can i do? Ive talked to the mom and she feels that they're kids and my son is as much of the problem as her daughter. But its to the point of where, he no longer wants to be outside if she is.Tldr: 3 year old moved in. Bullies my 5 year old who hates confrontation. Mother won't help. What else can i do? via /r/Parenting http://ift.tt/2qXL7RZ