My contacts are "offline," but I know they’re available. They can see me, why can’t I see them?

I have one contact on Messenger, linked with her Facebook, Twitter, and Microsoft Account. Whether I try to message her on my Windows Phone, Surface, or Windows 8 desktop, she appears offline, but I know she is available. She can see me as being available on her Windows Phone, and when she messages me, my phone suddenly sees that she is available, but only for about ten minutes. Her status then returns to "offline."

I have deleted her from my contacts and re-added her. I have synced my Account on my phone, and also gave her permission to see my details under Contacts via Outlook.com. Her status is available, and so is mine.