Except the OP mentioned to her MIL when the initial offer was made that SHE didn't want the stones but had a friend who would love them and MIL agreed to that. If MIL had an issue with the friend taking the stones instead of the OP, she should have spoken up then, not backed out later.
I think the MIL is completely allowed to back out later. They're her paving stones. And they can be worth a lot! And she was only in the planning stages, so it's completely reasonable that she might find, during the tear-out etc., that she has another plan for them. Maybe her landscaper clued her in to exactly how valuable they are, or something.
Or, maybe she agreed at that very moment without much thought. But in thinking, she decided something else. I don't think people must be held to those sorts of snap decisions. Even if she had some qualms at the moment, she might not have felt comfortable expressing them in front of the OP, who (perhaps) is enthusiastically forging ahead with her own plan for MIL's paving stones.
And the MIL may have thought her offer was sort of tentative anyway.
Of course that might influence how other people view her, and whether other people believe her when she promises something in the future.
(and it's one thing to offer something to your family member's friend but another to have the family member reassign it on her own. Maybe I'm projecting a bit, I'll admit, bcs I had once expressed to my soon-to-be-MIL that I was going to need to figure out what to do w/ my double bed when I moved in w/ her son, and she immediately said, "Oh, you can give it to this cousin or that cousin--they can use it!" and it really hit me wrong. It's my stuff; it does still belong to me, it's not yours to give away to someone else! If I'm going to give it away, I'm going to give it to someone *I* want to give it to. I didn't say much at the time, though I think I did say, "No, *I* will figure out what to do with it, it's *my* bed." But I felt churlish and rude for my reaction, and I hesitated to say anything. I can see me saying, "Oh, hmm," and her thinking I'd agreed.
And I can see that the more valuable something is--either financially or emotionally/symbolically, as was the case w/ my "I'm a grownup now" bed--the more reluctant someone might be to give things away at that "extra generation.")