My daughter's mom wants to see her

I have custody of my 13 year old daughter. Her mom was on a lot of drugs and left her at home with grown men she barely knew. My daughter ended up getting sexually abused by one of the men but my ex was so strung out on drugs and needed the guy for drugs she let it happened. My girl was only 4. I got custody of her when she was 8 and ever since her mom will randomly get clean. Try to build a relationship with her and then go back to drugs and its been a cycle. We haven't heard from her in two years. She messaged me saying she was clean and she's sorry shes been a bad mother but she wants to build a relationship with our daughter. My wife doesn't like the idea of letting her back in and when I asked my daughter she said "Is she gonna stay this time, or leave me again like she always does?" I don't want her to hurt. I dont want the whole popping in and out of her life. She could have changed and they could develop a relationship. Or she could just be talking, stay for a little while and leave breaking my daughters heart again. What would you do if you were me?