Does sin befall someone who described a married Christian woman as having "played" (meaning committed adultery) with a Muslim young man known for deceiving girls to reach zina? And what can be done to stop this young man from his actions?

If a woman is mentioned for a sin she openly commits with a young man, there is no harm in it, provided the limits of knowledge are not exceeded. However, if it is about something she does not openly commit, the preponderant opinion is that backbiting a dhimmī is forbidden. Women can be warned against being lenient in their interactions with young men, and the obligation of adhering to the limits of Islamic law can be clarified. Alternatively, one of her mahrams can be informed to advise her or to engage scholars and righteous people to guide her.