My friends mom borrowed money from her sister which she said she would pay back and her sister agreed on. However there is no paper on it nor a paying plan on the loan which she has paid back now but her sister still puts on interest on her "loan" and claims she hasn't paid it back yet which makes my friends mom pay ALOT more then she originally borrowed. Together with this my friends mom is verbally abused on facebook and bad mouthing her about her family and their money situation. This has been going on for months now.
My question is: Is there any way out of this? Is it really "legal" to put on interest and keep on harassing them on facebook etc or is it something they should/can do?