Unfortunately, we have lots of experience with this, and we live in California. It's the same as a foreclosure, basically. The bank, IF they choose to accept the offered purchase price, will charge off the difference. For instance, let's say the loan on the house is $200,000. Your friend has a buyer for her house who has offered $150,000. The bank will charge off the $50,000 difference, and your friend will take a ~200 point hit on her credit report. It will show up as a charge-off on a real estate loan on her credit report. She does NOT owe the money to the bank - to be perfectly clear, the mortgage documents she signed when she purchased the home indicated that the house was the collateral on the loan, and the bank agrees to accept the sale price as payment in full for the loan. They can't come back after her for the difference if there is only one loan on the property.
Two things to note:
1) If she has a second mortgage, or has ever refinanced the house, she needs to see a lawyer right now. There are some intricacies to the law that could make life very difficult for her - and she needs to make sure she is protected legally. If the second mortgage was used as purchase money when the home was first bought, she's fine and doesn't need to worry (the second "dies" with the house sale, even in a short sale, as does the primary mortgage) - but if she took out a second mortgage later on, or refinanced the first, the lender can and probably will come back after her for the loan. Different states have different statutes of limitations on that - in any event, she needs to get out in front of it and talk to a lawyer if she has a second mortgage, or has ever refinanced the initial mortgage.
2) If she is still able to make the payments on the house, it's very unlikely that the lender will approve the short sale. They really have no incentive to do so. At that point, your friend can do one of two things: 1) plan to hold onto the house, and rent it out, or 2) stop making the payments and allow the bank to foreclose. Foreclosure is the same thing on her credit report as a short sale, so it really makes no difference one way or the other to her. If she hopes to get a home loan in California, she'll have to wait 3 years before she will qualify for an FHA loan. So the option of renting out the house may be more attractive to her.
Good luck.