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 is His slave and Messenger.
It is permissible for your friend to take what her mother gives her even though her mother deals with Riba (interest and/or usury).
For more benefit on the ruling of eating from the money of the parents whose earnings are mixed with ill-gotten money, please refer to Fataawa 87211 and 81560.
Allaah Knows best.