There were multiple levels of failure here... TSA asks for boarding pass before screening, and the airline staff also ask for boarding pass and ID before you’re allowed to the aircraft, where usually you are again asked to show the boarding pass.
Someone allowed her to get where she needed to be to board that plane. She did not give anyone the slip. We need to know her ethhnicity. Most TSA workers are African-American.
Possibly got on via the food/beverage service trucks or cleaning crew. There are ways to slip through doors at airports....just saying.