If someone "disappears on you", it means that they suddenly vanish out of your life, even if only for a short period. It is used for things other than people. "iTunes was open on my desktop and then, just as I wanted to use it, it disappeared on me!"
Julie did not vanish into thin air in front of her boyfriend's eyes, which is what #2 suggests. They were together as a couple for a long time and everybody assumed that they would get married but then she mysteriously vanished. Presumably, she left the house one day to go to work but never came back (or something similar).
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