Is it better to give charity on behalf of the deceased by slaughtering and distributing meat to relatives and the poor, or by a discreet monetary donation given purely for the sake of Allah?

Charity on behalf of the deceased is desirable, and the deceased will benefit from it—Allah Almighty willing—whether it is monetary or in kind, such as slaughtering and distributing meat with the intention of charity. Both are permissible. Secret charity is better, unless public charity is better in some cases. Since the mother wishes the charity to be meat, we recommend obeying her and fulfilling her desire.