She is charged with a felony as a result of that prior Class C misdmeanor. Do not worry about filing an affidavit of non prosecution at this point. Help her get a lawyer and work with that lawyer. If there becomes a conflict of interest, then the lawyer can tell you so. At this point, you would have to work with the prosecutors to file the affidavit.
Check out the Texas Criminal Defense Lawyers Assocation site here: http://www.tcdla.com/source/members/creferralsearch.cfm?section=lawyerlocator
You can fill out the form and that should get you started in finding a local criminal defense lawyer for your partner.
Most lawyers know how to draft an affidavit of non-prosecution. Please understand that even if an affidavit is filed, it doesn't mean the charges have to be dropped. The state can proceed regardless. That's why you should hire a lawyer to assist you. Call me if you need my help.