One mother said that, after a meeting with Garza about her 5-year-old in which her own mom questioned the school's special education plan for the boy, Garza called Child Protective Services on her.
Garza's mom questioned it? Or the 5yr old's? Or the mother of the 5yr old's? Who is "her" referencing?
5-year-old, mom, and grandma showed up to the meeting with the principal. Grandma questioned the school's special education plan. Principal called CPS on mom.
You may want to reconsider your own reading comprehension.
One mother said that, after a meeting... in which her own mom questioned the school's... [plans], Garza called Child Protective Services on her.
Who is her and her own mom?
It is not specified at all.
You also completely failed to address the question at hand. OP asked who the "her" in "her own mom" is referring to. Your modified sentence doesn't even acknowledge the "her" in question.