@RedVanGhoul : No, based on her words she was tricked by the person she trusted (probably this guy) and something important to her was stolen, she IS the case of 'love interest betrays them'
Tiaana's case was 'fiance got stolen by another girl', we don't know if the fiance in question also betrayed her or was he simply fooled into following the scheme (the typical bitch heroine playing the victim to get everyone blaming the villainess)
So the likely scenario was the guy has been aiming for that precious thing, and in order to make a perfect crime has been slowly gaining her trust in him.
And that dungeon is his chosen crime scene.
Like, say it's a dungeon known for status inflicting snake, he intentionally let one hit her to paralyze her, stole the item, and then gloat on how her death will be seen as unfortunate but not uncommon situation, when the party is at risk of wiping the remaining members have to leg it and abandon their friends behind and such.