Court documents obtained by PEOPLE allege that Tonya Couch, 48, called her former husband, Fred Couch, days before going on the run with Ethan.
During the phone call, she allegedly "informed him that Ethan was with her and that he would never see Ethan again," the documents state.
Couch has been charged with hindering the apprehension of her fugitive son, with whom she allegedly fled with last month – soon after video footage was posted to Twitter showing him at a party where drinking games were being played. Had Ethan been drinking, he would have been in violation of the terms of his probation.
When he was 16, Ethan drove drunk and killed four pedestrians in 2013. He was spared jail time and sentenced to 10 years probation after his defense attorney argued he suffered from "affluenza" because of his privileged upbringing.
Police caught up with the mom-and-son pair late last month, detaining the two in the resort town of Puerto Vallarta after a weeks-long manhunt.
Ethan remains in Mexico, where his attorneys claim they might fight his extradition.
Jalisco St. Prosecutors Office
Police: Mother 'Has Voiced Some Displeasure' With Jail ConditionsTonya Couch's arrest affidavit reveals that before fleeing, "Ethan attempted to devise a plan to lie about the video and say that it wasn't him depicted in the video. Ethan was described as being scared that his probation would be violated."
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The affidavit alleges Couch withdrew the $30,000 in cash from her bank account days before she hit the road with Ethan.
Couch made her first court appearance in Texas on Friday, but did not enter a plea to the charges filed against her, PEOPLE confirms. On Monday, a judge lowered her bail from $1 million to $75,000; if she posts that amount, Couch must live in her son's home, a judge ruled.
A police spokesperson also tells PEOPLE Couch "has voiced some displeasure" with her current accommodations, telling law enforcement officials before her Friday court appearance that their jail "was too bright" and that she didn't like "that people were watching her."
"She spoke with the sheriff and he told her, 'You're in a jail, not a resort,'" explains Terry Grisham, a spokesman for the Tarrant County Sheriff's Department. "She's been there three more nights, so maybe she's gotten used to it."