If you have not personally struggled with an addiction, you do not understand how easily exposure to your triggers can throw your mind back into an obsessive state. Even if she does not start using again, it will severly set back her recovery process.
I also believe that her probation will be a bigger problem that you might think. I personally would use that situation to make your decision easier. She is on probation, cannot leave the state, and absolutely cannot be around others who are users. She will probobly have a probation officer by summer, who I would hope would not allow her to go out of state under those circumstances, and will probobly think badly for even purposing that she does.
If her mother wants to see your daughter, then she can get her ass on a bus and come to your state to see her under a supervised situation. Her mother cannot put that much effort into it, she can go to hell.
She has proven that she is incapable of putting your daughter's best interest first and has proven that she is in fact a danger to your daughter's mental and physical health. She is a drug addict. Drug atticts are not functional people. She may say she loves your daughter, but the fact of her current condition is that she really is not capable to loving your daughter at this point in her life.
If things have changed, then she can prove it by getting her ass on a bus.