HELP! House sold without knowledge for tax lien

[Megan Bridgette]

Hi, I have a friend who has s duplex in Georga.  She just found out her house got sold at auction to a company that buys tax liens, with out her knowledge.   The tax statements have been going to the wrong address so she has not been getting them.  She has 15 houses so I guess this one slipped her mind.  Is that lawful to sell her house with out even trying to contact her?  They bought the house for 56k, it is worth 75k.  In order to get the house back, she has to buy it for the 56k + 12k for all their fees. Even if she pays the money, they said that the persons holding the money (sheriffs office) might pay them back for the 56k and might not.   How is this legal???