Yeah if a friend spectates, they could be feeding info to the player and give them an unfair advantage. I think maybe giving the option to save a game after finishing and allowing other players to watch them like they were spectating would be better.
It's stupid, if you make it a minute delay, and no skill info such as cool downs it would be perfectly fine. LoL allows ranked spectating with a 3 minute delay and no one has ever won because of it.
CSGO delays the spectator footage by like a full round which is about 2 minutes on average. I use it to coach my friends live sometimes. Idk why OW couldn't do the same it's not like you can ghost someone from 2 minutes in the past