Sorry, but I think that was a serious offense. I personally might have given your friend a warning, but I'm not surprised she got fired.
I would allow my child to be given breastmilk from someone I knew, but not a stranger. This is no different, and my dh and I would be livid had it happened to our child.
And I can't believe people are saying this is all because it's breastmilk. Um, yeah... because formula, gross as it is, doesn't carry hepatitis and HIV.