I just got a reply from Tim (Ubuntu GNOME main Developer) and he said that is a bug and we need to file one.
So, I guess either I should go over her place and take the laptop or ask her to bring it to me.
I'm a bit worried that she might ask to replace the system since she said that she cares a lot about the HDMI and that is one of the most important things she always do with her laptop.
Tim is great and can solve bugs quickly but we don't know yet how long that might take.
Will keep everyone updated in case there is something new-There are known issues with most if not all HDMI monitors not reporting the correct physical size, there were some bugs (look for "Huge fonts" issue) caused by this, but I believe they are fixed now.
- There was a bug that affect cloning displays, but that should be fixed in mutter 3.10.4
-Some GPU's (Mostly Intel I think) have a maximum possible width, in this case side-by-side configuration won't work, but stacked vertically (top/bottom) will.
Whether the external monitor is HDMI or not, this bug will happen with any external monitor - see: http://ubuntuforums.org/showthread.php?t=2208614