Alternatively, perhaps I'm overcomplicating. Maybe the user is referring to a non-movie song, like a play or a live performance. The phrase "Woh Mangal Raat Suhani Thi" is part of a longer poem or lyric often recited in Hindi poetry circles or used in weddings. The full lyric might go something like:
"Woh Mangal Raat Suhani Thi, Woh Piya Se Chudne Wali Thi..." Alternatively, perhaps I'm overcomplicating
Alternatively, perhaps the song is from "Bhag Milkha Bhag" (2013) which has a song called "Mangal Suhani," but not exactly the same title. Alternatively, maybe it's from "Chandni Chowk to China" (2012) or "Bunty Aur Babli" (2005). Wait, "Chandni Chowk to China" has a song "Tamma Tamma" which is from a different era. Alternatively, perhaps I'm overcomplicating