Your client probably has the misconception that obfuscation will hide your code from prying eyes. You may want to educate your client instead that any code can be reverse engineered.
In that case, check out Obfuscator. It integrates with Unity’s build workflow so it’ll be less headache for you. There are other .NET obfuscators out there, but if it’s not really serving any purpose it should at least not get in the way for you. If he’s worried about cheating, he might want Anti-Cheat Toolkit instead. [edit: Fixed link - thanks, @Timelog ]