It’s not a drug so you’re not gonna have immediate effects. I would think you need to try it for a week and see how you feel.
Another thing I would look at is what your bro is REPLACING with celery juice. If he was drinking a bunch of soda and is now drinking celery juice instead, then most of his health benefits may be from cutting out sugar and caffeine. Energy wise, avoiding sugar will avoid crashes.
Let us know how you feel after a week. I’ve never heard of this and I live in California, but I don’t really put any stock into cleanses. If I want to clear out my bowels, I go to Taco Bell.