If you have the USCIS number e.g WAC, you can log into the uscis.gov website and check the status of your case, since you have mentioned that the case has been approved, you will find the approval notice there, you can even print it. However , i doubt another thing about this, that is, since your mom left the country after she applied, the WAC may not be valid. Although, since has a visa, she should not have any problem entering as INS allows 60 days grace to any overstay situation. all she would need is a plane ticket, if she has a valid visa.