You say there is no evidence against her. What about the property from inside the residence that was found in the car she borrowed (stole).
Whether the statements of an incompetent defendant can be used against him/her, will be decided at a CA Evidence Code 402 hearing which usually takes place at the beginning of trial. Being incompetent to stand trial does not necessarily mean she was incompetent at the time of her statements. If the statements were not coerced, voluntary and given after the appropriate warnings they may well come in despite her mental condition. Of course there is the possibility a good lawyer can keep them out.
It will depend on several factors. She should have an attorney defending her. Whether or not the statements will be admitted at trial will be determined by a judge at a Pre trial hearing. speak to her lawyer.
With regard to her post-arrest statements, should the case go to trial, there will be an inquiry into whether she made a knowing and intelligent waiver of her Miranda rights and whether her statements were voluntary or coerced, to determine whether they are admissible.
If her competency at the time of the questioning is an issue, there will be a hearing prior to the beginning of the trial where the judge will determine whether or not she was competent at the time of the statements and whether they are reliable enough to put in front of a jury.
SLG Criminal Law Group
515 South Flower Street
Los Angeles, CA 90071