Who send your mom the letter? The attorney, mortgage company, sheriff's office, or someone else who looked at a possible sale list and sent it?
The best way is talk to the landlord and see if he / she is doing anything to prevent the foreclosure. Ask the landlord to let you speak to the mortgage company so that you can get an idea of the time line.
If her lease is not up yet and she has been doing everything she does as the tenant, then don't rush, but take the time to look for another rental option or possibly buy her a home so you guys would be in more control.