How to appear online to someone on Facebook?

[That Guy]

A girl on Facebook was being really annoying, inboxing me every time I was online, so I made it so I was always offline to her, even if I was online. This was several weeks ago, and I want to know how to revert this (ie, so she can see when i'm online).

Thanks

[Jeanne]

click on the gear button of chat window >> select advance settings >> one new window will appear, you will see her name there, remove it and comfirm it.

or

From chat window, enter her name and click search then chat window will come, click on the corner and select turn on chat.