How can I help a friend who is lenient in conversing with young men and does not listen to advice, in order to protect her from following her whims?

You must explain to her the impermissibility of sins and the danger of committing them openly. Committing a sin is one transgression, and doing so openly is another. She must repent and conceal herself. Do not despair of advising her and connecting her with righteous company. If she responds, then that is good and excellent. Otherwise, there is no good for you in her companionship, lest she drag you into what she is doing.