Is it permissible to inform another party of what my friend did in order to help her, or should I conceal her actions? And if it is permissible to inform, who would that party be?

If a girl is not known for evil or openly defiant in sin, then the general principle is to conceal her actions. It is permissible to threaten her with informing her guardian to deter her, while continuing to advise her and avoiding her company. However, if she is openly defiant and known for evil, then it is permissible to expose her and inform others about her.

Her guardian or someone who can prevent her should be informed. Alluding to her behavior is sufficient if it achieves the purpose without the need for explicit declaration. When explicitly declaring, one must be cautious not to mention zina (fornication/adultery) as it may involve slander, and it is sufficient to allude when advising a suitor that "she is not suitable for you." And whoever is afflicted with sins should conceal their own actions.