I believe that feature is coming soon. Quite soon as there is a button for it already on the app that appears to do nothing yet.
Your friend can switch his mobile browser to desktop mode, and change servers and join a game that way. Once the game is joined it is easily accessed from the mobile mode
To switch to desktop, click the menu in the lower right, then the settings gear, and scroll down for the switch to desktop button.
I suggest leaving the mobile browser in desktop mode for such situations, and using the mobile app to play.
If he wishes to switch the mobile browser back to mobile, there is a button when loading the game just below the moving tank on the loading screen:
