Reporting it is the right thing to do. You pretty much have to. For one, the nurse who made the error needs to know to be more careful-- if this had been insulin, a seizure medication, vancomycin, a pressor... missing the dose WOULD have been a big deal. Even with Pepcid, if the pt's not responding to it, it may be because he's missing a dose, not because it's the wrong drug for him. Keep your reputation clean. And, as for the nurse who made the mistake... if she speaks to you, tell her calmly that your responsibility on night shift is to check orders, and that any missed order or missed dose automatically generates an incident report (or whatever form your facility uses). If she upbraids you for "telling," you can say something like, "I was thinking only of patient safety when I did, and if I had made an error I hope someone would point it out to me." It's not an issue of "telling" on your fellow nurses. We are here to support pts first and foremost. We also support each other... and think of it this way: she's better off having a small error found than later on down the line if she makes a big error.
Also, as a veteran of numerous JCAHO surveys-- these things are all eventually caught. Then you'll be asked why you didn't correct it, since you signed the 24 hr chart check. Protect your pt, protect yourself, and support your fellow nurses by helping them do the same.
Keep up the GREAT work!