True, few are thrilled at getting fired. But if you are told you're being let go due to performance they should be able to say how they determined your poor performance. Not being able to give metrics or manager feedback makes it seem like that's not the true reason. If you want to make fired people even more upset, lying to them is a good way to do it.
In this case, she made no sales at all in her first 5 months. At best, her performance cannot be determined. We could argue there was no data, but you could also argue you wouldn't keep sales with no data around.
People were being fired. They will be angry. There is no response that would make it better (of course, unless you give them more money).