So it would be nice to see your friend’s live ladder game.
So I propose an option, a request: can I watch your live ladder game right now?, and if your friend says yes for that particular game, you can watch it.
Just like in Dota 2, it will be fun, and plus you can learn so much from your friend.
these days we call that streaming 
but then does your friend get to view both sides of it? what if you’re in discord and they tell you what your opponent is doing?
or do we want them to have your vision only? because i could see a lot of people abusing the system if they had full map vision.
the only good way to do viewer games is to find a semi-popular streamer and get a bunch of people together who want to play a KOTH or something, i’ve done it a few times with people with the games usually being streamed / casted by the streamer.
Add a time delay and/or vision of what your friend can see
Oh yeah, absolutely time delay is a must!
Possibly just your friends vision or a big delay.