POA/Executor duties and property for sale question

I have a very dear friend I have known for many years and she and a sibling are 50/50 owners of property. Years ago before the bottom fell out of the housing market, she wanted to sell but her sibling wouldn't discuss it, so the matter was dropped (one of those people who has to be treated "special" and you must walk on eggshells around because they are always right).
Half of all expenses to keep up the house/property are shared.
Recently, she received an updated letter of expenses and there were charges for listing the property for sale in the local paper. She had no clue her sibling had even listed it for sale (she is several hours away and elderly and no longer visits the property). Needless, to say she was quite suprised, so I did an interenet search for her and found two additional listings through a realtor. One for a couple of years ago, the other for about 3-4 years ago.
She then receives a letter that a realtor wants to try to sell the property and her signature was needed. She inquired why it was needed now and not before and it ruffled feathers of the realtor and her sibling. The realtors response was, "well, your sibling is POA and the executor. They could take a fee if they wanted to but have not, so there." Well, that really didn't answer the question as to why a signature was not asked for before. Her sibling claims no other realtor ever asked for it.
So, I called a realty company that I have used before and asked them. They said if my mother and her sibling are joint owners, both signatures should be obtained to show the property and that my mother should have signed before.
I told my friend that I did not know if someone is an executor of an estate and a 50% owner, if they can put the property up for sale without the other person knowning.
I did tell her that there is a huge difference in what is legal and what is ethical and regardless if her signature was required or not to show the home, that I felt the moral thing to do was tell her.
Any thoughts on this what a person can do if they are executor of an estate? Can they just put property up for sale without notifying the other party?
Also, any thoughts why 2 realtors would not have required her signature and this one does?
I have bought property and sold property and never came across a situation like this. Thanks for any insight.