Boyfriend's birth mother showed up at our door! TW

I don't know what TF to do. His birth mother had tried to talk to me through social media months ago but I blocked her. I've never met her, I just know she fucked him up mentally. He was sexually abused and raped by her when he started going through puberty and didn't know it was wrong until he went to High school and then he tried to kill himself. He can't even look at pictures of her so he cut them all out. He's in therapy, but he still has panic attacks just from hearing her name. He was in the shower and I noticed this car had been drinking by our house slowly the past few days. He thought it was weird but didn't really do anything. I heard someone knock on the door. I answer it and it's her.... She asked if her son lived here. I said no and she said she knows I'm lying and that she is his mother and she's been trying to find him because she needs help. I told her to go away before I call the police for trespassing. She tried to hold the door open when I tried to close it and talk to me so I pulled out my phone and acted like I was calling 911 and she quickly left. It may seem really bad that I didn't tell my boyfriend but if I did he would go very far off the handle. Everything he learned in therapy would be thrown out the window and he would not be okay. I haven't seen that car anymore since then. But if I do again should I tell him?