You are able to file for divorce in Montgomery County even though she left, assuming the other jurisdictional qualifications are met. The law doesn't punish you if she leaves by making you go somewhere else to file.
The information provided is general in nature and is not to be construed as legal advice. No attorney client relationship is created through the questions an answers provided. Every legal matter is different and an accurate legal opinion can only be given after a consultation with an attorney.
You file a petition for divorce in Montgomery County. Have the citation issued and send it to the county sheriff where she is living. If you don't know where she is living, you must try to locate her. If you can't locate her, then you must cite her by publication. The court will appoint an attorney ad litem to try to locate her. Then you subsequently appear in court to "prove up" the divorce. That is what you do. If you can't figure out how to do all this, I suggest you get professional help to do it for you.
Lawyers are trained how to do this. You can't learn "how" on a website, anymore than you can learn "how" to do a root canal on your tooth.
I am not intending this to be legal advice, because I don't know the particulars of your situation. Call me if you would like to discuss this or other isues.
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