I have one in pokemon black
To evolve Riolu, it needs to be at max happiness and you need to level it up once in the day time. If he's at max happiness at night, great, but he won't immediately evolve in the morning. You have to level him up one level in the morning/day time at max happiness for him to evolve, but it doesn't matter when it reaches max happiness. Click here for more info and tips on evolving Riolu. ~atode!~ ('later!' In Japanese.)