This is actually wrong as well, the client-side does nothing more than presenting visuals for the underlying calculations which are done at server+client side at the same time, should they get out of sync, server calculations take priority. Think of it this way, server is the video, client is the player.
The problem isn't because calculations are done on client-side (they're not), there's a popular saying among developers: Never trust the client. If the game were client-side as you claim, I could simply tell the server that I've 3-starred every single opponent and it would register. Will it register if I tell that to the server? Definitely not, I get a shiny error telling me to Fk off.
The problem is because client has access to server data that it shouldn't have. How do you fix it? Don't give those data to the client until absolutely necessary. Simple as that.
PS: That guy pretty much explain why OP's idea would accomplish nothing but annoyance.
Can I wrap it up with a sarcastic comment, or should I explain why this is such a terrible idea as well...? Honestly, guys think before you make a suggestion (and be sober as well).

