All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
First of all, we congratulate this sister for her repentance, and we ask Allaah to not let her heart deviate after He has guided her to faith.
As regards your question, the answer is that repentance from a sin regarding the rights of the people necessitates being acquitted from this sin either by seeking forgiveness from these people or giving them back their rights.
When it is possible to know the person whom this sister owes any right, and it is possible to give him back that right even indirectly, then it is not permissible for her to give it in charity on his behalf. Besides, if she does not know the exact right that she owes to the person, then she should make an estimate that which she preponderantly thinks would acquit her from sin.
What you have mentioned that she is unable to contact them directly and she cannot find a Mahram (unmarriageable male relative) that could act on her behalf in giving them back their rights, then this is not an obstacle, as she may entrust a trustworthy person who is not a Mahram to her to do so, and she may put the money in the accounts of their rightful owners, or she may send it to them by post and similar means by which she may give back their rights and inform them that it is their property, but it is not an obligation to inform them that it is from her or inform them about the reason (of giving them this money).
With regard to the rights whose owners are not known, or that it was really not possible to make their rights reach them, and one loses all hope in making their rights reach them, then it becomes permissible to give their rights in charity on their behalf. For more benefit, please refer to Fatwa 90108.
Allaah Knows best.