@Hobart King typically all you will need to talk to the lender is a "3rd Party Authorization". It is simply a letter from the current seller (probably need to include the death certificate as well) explaining that she is the executor of the estate and that her mother, the mortgage holder, is deceased. She then will note the mortgage loan number and notes that that she gives you permission to talk to the bank about the any and all details of the loan.
You will then call the bank and ask for the Loss Mitigation Department. Let them know you have a 3rd party agreement and ask them if you can email or fax the form in so you can discuss the loan and the options for a short sale. Once you send it in, it may be a week before it actually gets filed into the system. Then you can call the Loss Mitigation Department back and ask to apply for a short sale. Hopefully by this time they have the authorization form in the system so they can give you direction on their process to complete a short sale.
My tips, call the bank every week. Most won't follow up with you, you have to be the aggressor.
Also, there will be a time when you call them and although you've talked to them 10 times about the file and the process, they will tell you they aren't seeing a 3rd Party Authorization form on file for you. Make a note of the date you send it in. It always seems that when I tell the specifically when I sent it in, they magically find it.
Good luck!