Is it obligatory to return stolen money to one's mother, knowing that she will spend it on unlawful things, or is it permissible to donate it as charity?

The money held by the mother: If it is her own property, then in this case she has committed two transgressions: aiding her son in drug use, and not being just among her children in giving gifts. She must, therefore, repent. However, if the money is under her guardianship for spending, then she has betrayed the trust and spent the money on unlawful things. She must, therefore, repent and return what she spent.

As for your taking of the money: if it was for a legitimate reason, such as the mother withholding a necessary expense from you, or not being equitable in giving gifts, then there is no blame upon you. However, if the taking was without a legitimate reason, then it is theft and impermissible for you, and you must repent and return the money to your mother.