france Yes, but maybe not for the reason you think.
I will assume B has no special obligations towards A (i.e. they are not a family member, they did not promise to keep the castle intact, etc.). Therefore, any liability is based on article 1240 of the civil code, requiring to prove (1) faulty conduct, (2) damage, and (3) causation between the two.
No non-tangible damage
At a first approximation, emotional damages do not exist under French law. Claims related to harm to a person usually require quantification, the most common unit being the days of "temporary inability to work", established via medical certificates. Most likely, A did not suffer such intense emotional distress that she could not show up to work the next day; and even if she did, B could not reasonably foresee that she would experience it.
If A establishes some other cause of damage, she might be able to claim "moral prejudice" damage. The closest common-law concept might be punitive damages: moral prejudice serves both to indemnify for "you did some uncool thing to me and I had to go through some hassle to rectify it", and to discourage future conduct. However, this requires some underlying cause of action.
The sand (probably) does not belong to A
Let us assume A does not own the land upon which she built the castle, and that she did not bring her own sand either.
The castle was constructed on a "public beach next to a lake". It is therefore likely that the sand was brought in at some point in the past, in which case it belongs to some public legal person (possibly the town council). Even if the sand was naturally occurring, it probably belongs to the owner of the land; "belong" becomes a more complicated notion that might involve mining permits etc. but I see no realistic interpretation where A can take possession of it unilaterally.
As a result, A cannot claim ownership in the physical object she created, no matter how laborious the process might have been. This makes prong (2) (damage) impossible to prove on the basis of physical destruction alone. It also creates difficulties for prong (1) (faulty conduct): it is well-understood that one of the purposes of a public beach is that people might play with sand, and B is therefore well within his rights to do so.
But A has intellectual property rights, specifically moral rights
The sculpture clearly is a "work of mind bearing the mark of the author" under the provision of the code of intellectual property. This gives right to authorship rights, which under French law includes moral rights. Those rights belong to the author (or their heirs, acting in the interest of the author) and cannot be ceded. In particular, a work is typically protected against alteration (which includes destruction).
That protection is not absolute; case law has long recognized that functional necessity might trump intellectual property protections, typically in the case of architecture. See this law review article for details. But that is irrelevant here, because B merely destroyed the castle for the thrill of it, rather than for some other legitimate purpose.
Therefore, A will prevail on prong (2) (faulty conduct). (3) will be easy enough, but she might have some difficulties regarding prong (1) (damage): violation of moral rights is a cognizable harm, but putting a monetary value on it is more complicated.
Not a whole lot to fight for
Under article 331-1-3, A can ask either based on the statutory criteria (monetary loss by A, monetary gain by B, moral prejudice), or based on an "all-inclusive sum" (which usually means "cost to reconstruct").
In Douai, 30 mai 2024, n° 22/01806 (the main case for the law review article cited above), plaintiff chose the latter option. That monetary value was set at €100k initially and at €62.5k on appeal. This was taken from the plaintiff’s estimate, produced on appeal, of the expense to recreate the work as close to the original as possible (*).
In A’s case, neither calculation amounts to much.
- 8 hours of manual labor, by a talented but presumably non-professional amateur, will at most be in the low thousands. A might try to argue based on her hourly rate, assuming she is a highly-paid lawyer / CEO / etc. in her day job, but I think this argument would fail (she could pay a low sum to an art student to recreate the castle).
- On the statutory criteria: B had no monetary benefits, and did not cause much economic loss to A if at all (she was not asking for payment for photographs, after all, so she would have difficulty arguing that she intended to monetize the castle). The moral prejudice is not going to be extremely high either.
But it is nonzero, so yes, A has a cause of action.
(*) I suspect some subpar defense on that point. How could the initial trial go for €100k of moral right violations if the plaintiff’s documented estimate on appeal is lower? Probably because the initial ask was some large rough estimate and the defense preferred to argue the principle rather than the amount. The appeal team tried to argue the amount, but used a cheap bid for approximate reconstruction; I see no universe where that works (if you don’t win on "was I allowed to break it down to remove asbestos", you’re not going to win on "am I allowed to reconstruct it on the cheap").
