It's expected that co-workers should look out for one another.
Jace overslept, so he's going to be late for work, but Colin can do his work for him in the meantime.
Colin doesn't want his boss to be mad at him.
Colin does his work along with Jace's in order to keep the both of them from being criticized by their boss.
Colin only does his work so that his boss isn't suspicious of anything that he's doing.
Their boss doesn't notice anything wrong at work, and Jace gets away with oversleeping.
Their boss notices that Jace is not at work after seeing that his work is left undone.