What is the ruling on advising a girl who has strayed from the straight path, and is it obligatory to persist in supplicating for her if she refuses the advice?

Denouncing evil is obligatory, and it is a Muslim's right upon his brother to be advised. One should strive to guide the girl and help her become upright through legitimate means, such as speaking on the phone, sending a gift containing impactful materials, or seeking help from righteous sisters to advise her. Meeting her secretly is not permissible due to the forbidden seclusion involved. If she refuses advice, then you must supplicate for her guidance, continue striving to guide her, and persevere.