A lot of the advice seems to be taking the tack, "Don't lose your cool" or "Don't upset the customers". The OP already stated that the company has already upset the customers. The circumstances surrounding the upset, the company's policies on dealing with irate clients...we know none of that. Telling her to relay to them, "Don't get a bad reputation in the first place" isn't going to do much good. That horse has already left the barn.
The best thing to do is bite the bullet, own up to any mistakes that were made on the company's part, look hard at their policies and procedures and change them so that this mistake has a minimal chance of happening again.
It may take years for their reputation to clear from this upset, if it ever does. However, I personally have been willing to work with companies who say, "Okay, we messed up - here's what we're doing to make sure this doesn't happen in the future". They realize that mistakes happen, that many of them are preventable, and if it's within a company's power to prevent or avoid mistakes, the company should do what people should ideally do: learn from their mistakes and move on. Sadly, though, there's no "magic bullet" or "quick fix" once a reputation has been damaged, even if the company were not at fault. How long the climb back, depends on exactly what happened and how the company reacts to it.