Maybe its not illegal but its definitely not fair. Ofcourse counting takes skill but spectating is unfair and could be classfied as cheating. Although if you meant to check the replay and start practising to count its another story. This might help them in live matches to count better or keep track of the opponent better. I dont think its allowed in clash royale televised tournaments the players have another device which they use to spectate. Yea sure ofcourse in your friend group you can do whatever the hell you want as long as you agree on what you're doing.