Is it obligatory to inform the girl of what this young man did, or is it better not to interfere, knowing that there is no relationship between the questioner and the girl, but there is a friendship or acquaintance between the questioner and the young man?

It is not permissible for a woman to befriend foreign men (non-mahram), nor is a relationship between them permissible except through a lawful marriage. If a man returns to a girl whom he has been toying with, intending to propose to her, and you know of a deficiency in his religious commitment or character, you must advise her to leave him. This is not backbiting but rather sincere advice (naseehah). However, if he returns to her to continue an illicit relationship without a proposal, you must advise her that such relationships are forbidden and must be severed.