It's actually Jujube (พุทรา) like your bro said ka. Many Thai restaurants sometimes make Rosella Juice (นํ้ากระเจี๊ยบ) mixed with Jujube 'cuz theirs taste give a good combination. And having med properties as herbs at the same time.
As I know in traditional medicine method, the Jujube can remove toxins or reduce bad-side-effect from Rosella. Then, Jujube is always boiled with Rosella.