I don't think the realtor can put a lien on a property unless the property owner OWES the realtor money, and in this case, I don't believe that deal would create any indebtedness on the part of the home owner.

There is something else called a "caveat" that attaches to the property very much like a lien. A caveat is like a clause that get grandfathered in if there's any change in ownership of a property. For example, a father might sell an apartment block to his son with the caveat that the father may live in one of the apartments rent-free for the rest of his life. In that case, it doesn't matter if the son sells the building to anyone else, that third party has to honour that caveat and allow dad to live rent free in one of the apartments.

Similarily, if the son gets killed in a car accident, and ownership of the apartment block passes to the grandson, then the grandson has to honour that caveat by letting gramps live rent free in one apartment.

Caveats can be as bizarre as you can imagine and aren't set up primarily to extract money from the homeowner like liens are.

I expect that in your friend's case, the realtor is putting a caveat on people's property saying that they can't sell it unless that realtor handles the sale and gets the commission. And, that would be a valid caveat.