The ड़ is a cerebral sound, and so when pronounced properly the "r" sounds like "d". Hindi, derived from Sanskrit, would also follow the rule of the 5 mouth positions, including gutteral (back of the throat as in "k" or "g"), palatal (top of the palate as in "ch" or "j"), cerebral (smooth, round area of hard palate behind and above the teeth), dental (touching the back of the teeth, as in "t" or "d" or "n"), and labial (lips, as in "p" or "b" or "m").
To me it sounds like 'rl', or at least like a slapped 'r' at the end. I think 'd' is often written because ड़ looks like ड, which is already written 'd' for more obvious reasons. Though I never heard somebody say लडका, but could it be that some non-native speaker picked the word written in Latin characters? I know someone who says पैसा, पाइसा, I wondered if it was because it's written paisa in Latin characters or something else.
This problem is interesting to me too. This sound ड़ is pronounced like “rl”, like soft “d”, like soft “r” or maybe like French “r"? In Duolingo, it sounds in different words in different ways. And the same problem with ढ़. I have no answer. Maybe someone can give answer to this?