Step 1: Add your co-worker's email onto your Folder Pane. Once done, go into their inbox, and select a message that needs replying to.
Step 2: Double click the message you wish to reply to, once opened, go to the Message tab > Reply.
Step 3: When you click reply, the From field indicates that you are sending the message on behalf of your co-worker. Type out your message in the message box, and when done hit the Send button.