You can watch, in Observer Mode, the matches of your friends only.

To watch a match, open your friends list and find the button with an eye to the right of the friend who is playing (see the red arrow in the image below)

By clicking on the eye button, you will see the table where your friend is playing and be able to follow the game.

Your friend will get a notification asking her if she agrees that you see her cards.
If she accepts, you will see her cards while she is playing.