A friend told me that he romanced Leliana and then chose to die in the game. And then he said he regretted romancing her, because Leliana commited suicide in the epilogue. Did he lie to me?
Kind of. The epilogue can be interpreted that way -- it's what came to my mind, too, when I saw that slider in my game -- but it is not actually stated as fact. Here:
“Leliana remained in the royal court for a time, mourning. She poured her heart into a ballad that would eventually become known throughout Thedas. But after one performance, Leliana quietly vanished. Some say that the Maker came to her in a vision again that night. Smiling, tears in her eyes, she told a maid that she would see her love again at last.”
Since the Warden is dead, that can be read as implying that Leliana is planning to off herself. (On a tangent, I'd have liked to hear that ballad in Inquisition. Or at least a mention of it, if it's so famous.)