I don't know, but one solution (since you have access to her account) might be to change her password to something she would not be able to readily guess. And keep her off for the 48 hour period - partly to be able to deal with the re-blocking issue and maybe also as a small punishment for unblocking someone you don't approve of. Then give her the password once the re-blocking issue is resolved and you have extracted a promise from her that she will respect your wishes when it comes to blocking people.
FB is a privilege and not a right. I predict that she will squawk about this. But I think you should get to the root of this problem. Otherwise, it's just going to keep happening, over and over again.