Didier evidently wrote a book, which can be downloaded, published in 1867 as
Le sommeil magnétique expliqué par le somnambule Alexis en état de lucidité by Alexis Didier. That is, "Magnetic Sleep, Explained by the Sleepwalker Alexis While in a State of Lucidity."
Charles Dickens once went to see a show staged by a clairvoyant of that name, presumably the same person, although Dickens describes him as "Belgian" and as a "Magnetic Boy". Dickens was impressed by his seeming clairvoyant powers.
The alleged supernatural healing powers of magnetism, as claimed by another practitioner, Mesmer, were investigated by a Royal Commission in France not long before the Revolution. Benjamin Franklin was a member. THe Skeptics' Dictionary states that:
A committee of scientific investigators, including Benjamin Franklin (1706-1790), found that the energy healer did have successes, but they were due to self-delusion or illnesses running their natural course. The committee found no reason to postulate magnétisme animal or any life-force manipulation to explain the satisfied customers.
Franklin observed that "magnetism without imagination produces nothing".
I suspect that Didier the "magnetic boy" was likewise devoid of supernatural powers, but it is open to anyone who thinks otherwise to produce the appropriate evidence and impart it to us.