"if the OP's MIL said, "Oh, okay," is that a promise?"
This seems to be dancing around the point. According to the OP, MIL agreed, and that's enough of a commitment that backing out requires positive action on MIL's part to advise lemonfloorwax/friend that she's changed her mind. Changing her mind isn't the fault I see here, it's doing so without then telling anyone that the change occurred.
Audrey Quest wrote:
"Nowhere is the MIL asked, are you sure you want to get rid of those? Are you sure its ok if my friend takes them? That should be the attitude. Not, "OK, cool, tell us when.""
I can't get comfortable with considering the sort of reaffirmation to be universal. According to the OP, lemonfloorwax's "MIL agrees and says she will let me know when I can come get the stones." I think it's reasonable to take MIL at her word when she agreed, and so I don't see where any further request is necessary. As long as lemonfloorwax's friend expresses her gratitude she's well within bounds.
"Also, how could the friend make arrangements to pick up the stones if she hadn't even talked to the MIL to find out when it would be a convenient time to do so?"
Arranging transport can be a multi-step process. Just yesterday I did something of the sort for someone who wants to pick something up from my yard that's very big and heavy. We initially discussed moving it, and then he found someone from whom he could borrow a truck. After that arrangement he called me to figure out when he should borrow it to get the item. I see this the same way, so I don't see any overt ball-dropping in this.