My daughters mother is addicted to pain killers and I can't talk her into getting any help. What can I do? ?

My ex and I have been split up for over a year now and we have a adorable 4 year old little girl. I have custody and the mom hasnt seen her in 10 months. The mom has contacted me recently to see her but I wont allow it.. She is shooting up and has track marks (bad). I dont want my child to see her mom in the shape she is in.. I have offered to pay for rehab and everything else I can for her. When do I just give up.? I want her to get help but I am at the end of my rope... I love her dearly but when do I say enough is enough?