As you know, we cannot control them. You can, however, make it clear that if she leaves early, she will not be staying with you. The sooner she knows that, the sooner she can either decide to stay in rehab, or make other arrangement for a place to live.
Also, you are NOT her only support. There is plenty of support out there, but she has to be willing to give it a chance. The fact that she's wanting to leave rehab early tells me she isn't truly serious about being sober.