Jesse's Girlfriend is still alive. The poisoning attempt was to shake Jesse's confidence in Gus. Gus was wooing Jesse to cook so Gus could get rid of Walt. However Jesse remained firm and kept telling Gus he would NOT cook if anything happened to Walt.
Walt either did not believe Jesse would keep faithful to Walt or did not realize Jesse was in fact standing up to Gus and refusing to cook if anything happened to Walt.
I think the poster was referencing Jesse's earlier girlfriend Jane from Season 2. When Walt breaks into their apartment to recover some money, he sees Jesse and Jane in bed after a hereion binge. Walt notices as he is leaving that Jane is choking on her own vomit, but chooses not to save her.
Many thanks for connecting the dots.
This is one show that really keeps you guessing.
Aren’t many shows with real good writing like Breaking Bad, Justified, Suits, Major Crimes and The Good Wife. And I miss House.