Wrong, wrong, wrong.

Eh, it definitely has a cause. A known one.

Nothing to do with the physical definition of spontaneity. Spontaneity of a process just means that the ∆G is negative or total energy of the system is lower after the process, and additional energy isn't required for the process to be thermodynamically allowed. This is, and I can't stress this enough, the simplest of simple thermo.

for the sole reason which specific molecules is nigh impossible to predict

Also unrelated, but it is fully impossible to predict, since in trying to predict it well enough you reach quantum scales where everything is probabilistic. That doesn't at all mean everything is spontaneous.

So, who is correct depends entirely on the mental framing of what someone thinks of when they read "water".

Nope, the first person is strictly correct and the second is strictly incorrect, as described above.

Water as an abstract idea of a specific type of fluid? Not spontaneous.

Nope, exactly spontaneous. You could even forget about water entirely and model this just as a bunch of nuclei and electrons in a box and derive that the lowest energy state has them being in a gas of atoms, and the initial state doesn't, which is enough to demonstrate by our earlier statements that boiling is spontaneous.

Water as in what will literally happen to the bottle of water in the picture?

This is "not even wrong" territory.

This post isn't showcasing mansplaining.

It absolutely is. We will define mansplaining here as the confidently incorrect dismissal of statements of women by men where we suspect that the genders of the participants may play a role.

The first part has been demonstrated above. It is also reasonable to assume the second given that we observe this happening to women at a far greater frequency than to men. Although, like with atoms, we cannot prove that this individual instance is a direct result, it is consistent with the probabilistic data and we would need additional evidence to conclude that this particular guy just goes around wrongly correcting everyone equally.

Nearly valid pedantry at that.

Once again, not remotely.