It means they're onilne with a mobile device, If your friend use's an iphone it probabply, the symbol will stay there even when he's offline, just cuz no one logs off their iphone...
If you are currently online, it will display the green dot
If you are not online, but have a FB app installed on a mobile device, it will show the mobile phone icon, and messages sent to people with this icon will be recieved as messages on their phones but it doesnt mean they are online