the body of the article clarifies that the 16-year-old girl told police that Mr Yadav had asked her to marry him before the alleged rape:
"The girl said Yadav had promised to marry her and then... raped her," said the [Ramol police station] official.
Because the impregnation (an alleged rape) occurred after the proposal, using the pluperfect or past perfect ("...had promised...") is an improvement that communicates more information. Expressing the idea as "...had earlier promised..." is clearer still.
Why didn't the writer say it that way? If it were in a headline, those are usually written or touched up by editors who might shorten the phrasing for effect or to fit the column width. The sentence here is part of the story's lede and there's no real reason to phrase things ambiguously. The numerous serious grammatical mistakes in the anonymous article suggest it was just one of a series of careless errors.
That said, using the past simple ("...promised...") isn't wrong in this context. The grammatical detail is irrelevant to the accusations retold in the article. Whether the proposal occurred before, after, or during the rape has no relation to whether a rape actually occurred or not. Because of that, the phrasing "...who promised..." doesn't impact the meaning of the sentence. It just renders it somewhat less informative.
This is the person who helped your father.
is grammatically correct. This person completed the action of helping your father at some point before now.
This is the person who had helped your father.
could be correct if there is more context lying around. This person completed the action of helping your father at some point before something else (currently unmentioned) in the past.
It was broken by the person who had helped your father.
is correct, so long as he completed the help before breaking the object. The grammar is timing the help relative to the breaking of the object instead of to the present.
It was broken by the person who helped your father.
isn't wrong either. It is just timing both completed actions relative to the present, leaving their relationship to each other unclear.