To play devil's advocate, it is sometimes easy to misconstrue the emotional intent of phrasing in email, especially when you have preconceived notions based on previous experience. She may have simply been saying (albeit awkwardly) that there was no issue with the cake, and that you did your best in dealing with her emotional state.
That said, I would abide by the MOB's wishes and "leave it at that".
Ah, Jason, you're giving this woman too much credit! I think that when someone says "I'm sure you did your best" there's no mistaking the passive-aggressive meaning. She's trying to get a reaction. OP-Don't let her even though I know it's killing you to know what actually happened!