Nothing can prevent litigation. You can make sure in the future, you don't enter into oral agreements. For now, best thing you can do now is put your agreement in writing (that she agreed to move out). Get her to sign it.
Next best thing, summarize your agreement with her that she move out in a letter you send to her certified mail, RRR.
Finally, don't sweat it--its unlikely she will "sue" you--the vast majority of individuals in such a situation don't.
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