Miriam,
It is some sort of Crinum by the look of it, but I have no idea how many would be from that area. It looks like the Crinum pedunculatum that is common up north in Australia, so maybe they extend that far up as well into Thailand? But I think there are a number of Crinum from that way, at the very least including Crinum thaianum. Maybe someone with local knowledge of what grows in Thailand can help?