Has the tree stump been damaged ? that would be the only 'comeback'. A driver is supposed to look where they are going even in reverse and avoid things like tree stumps. Suppose instead of a tree stump it had been a small child sitting on the verge at the side of the road,( happens every day in my road )
I am sure there are no regulations about the height of tree stumps , you just have to look for such things. Tell your client to claim on his/her insurance if the ca is damaged and to look where they are going in future.