You should always help out your coworkers.
Megan coworker's is struggling on a project which she has expertise with and has asked for her help during work hours.
Megan wants to finish her own work on time.
Megan makes some time to help her coworker in exchange for some help in return with part of her work.
Megan refuses to help citing that she has her own deadline to meet.
Megan's coworker learns how to complete her task and the project goes on without issue.
Megan's coworker is written up because she struggles with the project.