Is a HOA foreclosure legal without notice?

A paid-off property in my family was foreclosed on by an HOA, without due notice. We received an eviction notice from the ‘new owners’ yesterday when we stopped by to check on the empty place that is being shown. We have such little time and any help would be so appreciated. My Grandmother lived in and bought her condo 15 years ago with cash. Grandma died in 2010. Her son, my father, was the Power of Attorney in charge of Grandma’s estate. Dad was diagnosed with cancer in 2008. When he received notification from the HOA that Grandma’s property would be in foreclosure due to owing one years worth of HOA fees, he was weeks away from passing and was not coherent, so no one knew this was happening. The HOA says his spouse signed for the paperwork, as she was with him along with his children, providing hospice care to him in his home. Dad died in February, 2011. In about January, 2011, the condo was placed on the market by my father, and was being shown to many potential buyers. After dad’s death, my sister became the Personal Representative of Grandmas estate in June, 2011. This was after the first notice was sent in February, but before the foreclosure auction of the property which happened a few weeks ago. My sister was not aware that any fees were due, and was not receiving any correspondence from the HOA, because it was being sent to my dead father (The HOA personally knew my father and was aware both he and my grandmother had died). My sister continued with the same realtor and proceeded to show the property through all of this. The buyer as well as the condo community came into the apartment, saw it was being shown, also used Open Houses from our own realtor, listed it themselves, then finally auctioned it off, all without every telling the realtor that frequently would talk to them as she came into the building to show the unit. On August 31st, we discovered a note on the unoccupied unit saying there was a 3-day notice to evict.