I found contradictory information on this. A biography of Lavoisier which only suggests that Fourcroy failed to step up in his defense when he most needed it. See:
Jean Pierre Poirier Lavoisier: Chemist, Biologist, Economist On Google Books
On p367 in the chapter "The Arrest"
These interventions [on Lavoisier's behalf to counter the charges against him] belie the claim that Lavoisier was simply abandoned by his colleagues and that nothing was done to try to save him. It is no less true, however, that the four men, Monge, Hassenfratz, Guyton de Morveau, and Fourcroy, who were best placed to rescue him, since they belonged to the majority of the Convention, did not speak up in his favor.
Prior to this though, p336 refers to the continued interaction up to the arrest though "even if their political convictions separate them," but later notes that "Fourcroy insisted on a purge to get rid of the counter-revolutionaries, the emigrés, and all those whom public opinion rejected" even if others that might fit Lavoisier's description, that could make "the Lycée républicain a new and regenerated institution" should be spared.
There were other occasions when Fourcroy did have a role in someone's release, as in the case of Vandermonde, arrested for corruption, and Vandermonde was subsequently released (p332).
However, elsewhere, there is a claim that Fourcroy "pleaded for Lavoisier's life":
Arthur Donavan Antoine Lavoisier: Science, Administration and Revolution On Google Books
On p295-6 chapter on "Revolutionary Politics":
When Fourcroy heard that the tax farmers were to be arrainged before the Revolutionary Tribunal, he boldly pleaded for Lavoisier's life before the Committee on Public Safety, of which he was not a member. Robespierre heard him out without comment and Fourcroy, whose own life was in danger, was unable to prevent what had become inevitable."
This claim of an "unsuccessful last-minute appeal" is also made here:
Encyclopedia.com Entry for Fourcroy
The refutation by Baron Cuvier mentioned on Wikipedia can be found in:
Sarah Lee ed. Memoirs of Baron Cuvier On Google Books
It was reported that he might have saved the life of M. Lavoisier during the reign of terror, as indeed he had saved many by his influence; but, at the moment of M. Lavoisier's arrest, his own life was threatened, and all power of being useful to others was taken from him...
Another source on this is:
Denis I. Duveen "Lavoisier Writes to Fourcroy from Prison" p59 On Jstor
[Lavoisier's] standard biograph, Edouard Grimaux...strongly condemned Fourcroy for allowing Lavoisier to be sent to the guillotine and implies that, possibly motivated by jealousy, he may have helped to speed himon his way. Modern scholars are inclined to be the opinion that Grimaux maligned Fourcroy unjustifiably. The charge, however, was evidently current shortly after Lavoisier's death, for in a speech delivered only two years after the lamentable event Fourcroy felt constrained to defend himself against and accusation which was to haunt him for the rest of his days and pursue him from his own death until the present day.
It then quotes a letter from Lavoisier to Fourcroy from prison in French, which unfortunately I don't read but publisher of the letter doesn't conclude in a way that suggests that it settles the controversy.
Charles Coulston Gillispie Science and Polity in France: The End of the Old Regime On Google Books
An unpublished memoir by André Laugier, a cousin, gives more detail [on Fourcroy's attempt to save Lavoisier]...on one of the two days that the farmers general languished in the Conciergerie, Fourcroy burst into the room in the Pavillon de Flore where the Committee of Public Safety was meeting...set forth in a passionate and eloquent manner what an appalling loss to science Lavoisier's death would entail...but Robespierre's reaction afterward was so menacing that Prieur followed him to the corridor and warned him never to say more if he valued his head. [Sourced to: "Notice sur Fourcroy, écrite par une personne de sa famille" in hands of Cuvier and found in Bibliothèque de l'Institut de France, Fonds Cuvier, Carton 1, dossier 191]
This Laugier account is repeated in Madison Smartt Bell Lavoisier in the Year One p182 On Google Books