Don't let your boss take advantage of you. Read your employment contract. Remember, you have a job description and a contract and if the boss tells you to go beyond or even sideways of either them, they are taking advantage of you.
On top of this, they can use you as a scapegoat and blame you for the coworkers actions. Be patient and be strong! Speak to your coworker about this dynamic, and how you are neither trained nor paid to train others.
DO NOT DO EXTRA WORK just to be the fall person for when things go wrong. DO NOT DO EXTRA WORK just to be doing extra work. DO NOT DO EXTRA WORK to fill the gaps your employer consciously leaves open.
DO NOT DO EXTRA WORK. DO NOT DO EXTRA WORK.