Can the devil provoke a mental illness?
If the devil can tempt, he can also do it continuously, intensely, without rest and try to provoke an obsession, phobia, depression or other ills. If we ha said that he can transmit intelligible species (ideas), he could transmit them with such frequency as to seriously perturb the ordinary life of a person to the point of unbalance. So, it is something the devil can do. However, God impedes his free actions over us. Any action on the devil's part concerning humans has to be permitted by God.
So the answer to the question as to whether the devil can provoke mental illness is: yes, if God so permits. This answer is valid for everything, including the question about whether we can contract a mental illness without the devil's intervention. The answer is the same: yes, if God so permits. This is an answer that has a certain universal character to it. But however general it may be (-in fact, it's valid for everything-), I am afraid there is no other answer to this question.
Considering that we know the internal mechanism that the devil uses to provoke temptation (-the infusión of intelligible species or ideas in our mind, memory and imagination-), this "modus operandi" can also be used with such insistence that it can cause mental unbalance in a person. It is within the devil's power to be able to do so. The only thing that can impede it is God's will. But, does God impede it always? Undoubtedly not. If God does not always impede the natural causes of sickness, He won't always impede the devil's action either. However, in this field of causes of physical and mental sicknesses, the devil's action is exceptional. All mental illness is due to natural causes as long as the contrary cannot be demonstrated.
On the other hand, if we put, side by side, a person who had a naturally caused mental illness together with a person whose mental illness was caused by the devil, there would be no way of distinguishing them, since we only see the external effects.