。
She said goodbye to me, and that was the last of it.
Literal
She [topic-は] me [to-に] that [object-を] [as last-に最後] goodbye [object-を] said.
それを最後に is a fixed adverbial frame: 'with that as the last,' meaning 'after that, no more.' The structure [noun]を最後に marks a final occurrence — used for last meetings, final words, last events. The whole expression hits a note of finality — she said goodbye, and that goodbye turned out to be the genuinely last one. さよなら itself is the warmest, most universal Japanese farewell, but it carries a hint of permanence — Japanese sometimes prefer また or じゃあね for everyday partings, reserving さよなら for genuine departures.