La Voisin's a kindly elderly peasant who doesn't think twice about taking in two bedraggled strangers. He doesn't have much, but he offers it all up on a silver plate to Emily and St. Aubert. Of course, St. Aubert recognizes that La Voisin's generous nature is typical of "French courtesy," but not everyone would bring in the drama of a dying stranger (1.6.40).
When it comes down to it, La Voisin just likes to get nostalgic about his past life with his wife and children. Sound like anyone we know? La Voisin may just be the perfect foil to St. Aubert, whose own life is drawing to a close faster than he knows. La Voisin also asks St. Aubert some pretty interesting questions that Em will reflect on long after her father is gone:
"Do you believe, monsieur, that we shall be permitted to revisit the earth, after we have quitted the body?" (1.6.44)
Sounds a little creepy, sure, but La Voisin helps Em come to terms with her father's death.