I'm going against the grain here and vote to keep quiet about it because, like other posters have pointed out, you do not know what the situation was before you overhead the yelling manager. In some instances you going to a higher-tiered manager in the store, or even the corporate office itself, may be viewed as a customer complaint about the young employee and make the situation worse then it already is. In some instances upper management and corporate management can't seem to understand the concept of "[manager] was very rude to [employee], was berating them for [issue] very publicly, and it's not good business for your managers to be yelling at their employees like that" and might view it as "[employee] did something bad enough that even a customer complained about [employee], fire them!".
Not that the manager was in the right for yelling at the employee, far from it, but sometimes it is not always best to speak up in defense of an employee. The example I gave above may be a long stretch in most companies, it really isn't a hard concept to believe at my current workplace, so it might not happen. I'm just saying that it might not be in the best interest of the employee if you do decide (or already have decided) to contact someone higher in the food chain.