Your boss shouldn’t really tell you what your co worker needs or doesn’t need to do. It’s bad practice.

I’d not worry about other people at work, just do your job the best you can and don’t sweat small stuff.

I’d also be careful about complaining about being overworked. If you are salaried it doesn’t really matter how many hours you need to spend on your task as long as work gets done. But if you are hourly, they need to pay overtime. Other than that it’s what it is. Bosses don’t usually care on any kind of deep level if you feel overwhelmed