Will I be sinful if I give my friend my phone charger for her to charge her phone with it, and then she uses the phone for forbidden things?

There is no harm in lending a charger for charging a mobile phone, even if it is subsequently used for unlawful matters. This is because it is assistance in a permissible act (charging the mobile phone), and not direct assistance in a potential future act of disobedience. Furthermore, the fundamental principle regarding a mobile phone is its use for permissible things, and its use for unlawful things is an incidental matter upon which a legal ruling is not based. However, if the lender becomes certain that the purpose of borrowing and charging is to commit a sin that depends on it, then it is not permissible for him to assist at that point.