Simple suggestion, on the player menu when you click on someone name, the one with invite, profile, message, friend, block, report. On that menu, add a new button, greyed out if said player isn't in a game. When this button is clicked, you get sent to spectate the battle said selected player is on. This would be a much easier way to spectate a friend game, instead of going mad scrolling the "games in progress" in the multiplayer section. This could also provide an avenue for spectating single player battles, which could be nice ( potentially opens up the possibility of doing pve tournaments ! , referee spectate to ensure no cheating the result etc etc)
This would definitely be a useful feature, especially now that there's more than ten matches going on at any given time.
I think there was a topic created in the past about this, with the suggestion of something to click next to a player's screen name? That might be even more easy to use than something in the menu.