Where did this person get the blocks, may I ask? Just curious how a non-sewer came upon a boon of a mountain of blocks!
I can't reiterate enough what others have said. SET A time for a cost discussion BEFORE you even begin. Not doing so could cause some true hard feelings. And with family? Mercy. That's one I'd not want to have.
You are kind to consider it. Maybe calculate the actual cost and time involved and ask HER what she thinks is fair. When she sees what the cost of material alone is, she might have a better idea of what is an agreeable amount. And then renegotiate if need be
Good luck!