Monsieur Forez, the official Hangman of the 5th Arrondissement in Paris of the 18th century.

In 1744, Monsieur Forez meets Claire Fraser at L’Hôpital des Anges when she helps him set a badly broken leg. Monsieur Forez then gives Claire a ride home in his carriage. Monsieur Forez gives Claire a jar of hanged-man’s grease. Monsieur Forez calls on Jamie Fraser and Claire with the outward excuse of delivering a herbal package to Claire from Mother Hildegarde. His real purpose is made clear when he gives Jamie and Claire a graphic description of exactly what occurs when a traitor is sentenced to the traitor’s death of hanging, drawing and quartering.

Monsieur Forez was based on a real public hangman in Paris of the 18th century.

While many executioners practiced their medicinal trade quite legally, there were others that didn’t. Executioners were often challenged by more reputable surgeons and doctors, and many broadened the scope of their practices. It’s been well documented that Pierre Forez, an executioner at Lille, France, also dabbled in the sale of “hanged man’s grease” for various medical practices.
This hanged man’s grease (also called “poor sinner’s fat”) was fat taken from the corpses of hanged men. It was believed to work as a salve when applied to limbs suffering from lameness or restricted blood flow, arthritic joints, and it even aided in the mending of broken bones. It was, of course, a commodity that executioners had access to in quantity, and allowed many of them to work as pharmacists and apothecaries as well as doctors and surgeons.
Other body parts salvaged from executed criminals were also used in medicinal practices of the era. Ground human skull was mixed into drinks for those suffering from epilepsy, and it was believed that “poor sinner’s blood” could also cure the seizures of epilepsy. Human skin was tanned and made into belts to be worn by pregnant women to ease labor pains or chokers to be worn for the prevention of goiters.

sources:
https://knowledgenuts.com/…/the-executioners-who-practiced…/
http://outlander.wikia.com/wiki/Monsieur_Forez
Matthew Ramsey. “The Old Regime.” |”The Old Regime.” Professional and Popular Medicine in France 1770-1830
Image from Starz

Thanks Susan Mclaughlin for your help!