In Albert Camus' The Stranger, it is clear that when Maman was living with Meursalt that they were not close. Meursault admits:
When we lived together, Mother was always watching me, but we hardly ever talked.
After Meursault puts her in Home of the Aged Persons in Marengo, fifty miles away from Meursault in Algiers, she becomes close to elderly people her age. The doorkeeper admits to Meursault:
“She had good friends here, you know, old folks like herself, and one gets on better with people of one’s own generation. You’re much too young; you couldn’t have been much of a companion to her.”
In particular, an unnamed woman at the vigil, seated in the second row, weeps more than the other women present. The keeper tells Meursault:
“She was devoted to your mother. She says your mother was her only friend in the world, and now she’s all alone.”
Also, it seems Maman had a gentleman who was fond of her, Thomas Perez. The keeper says:
"But in this particular instance I’ve given permission to an old friend of your mother to come with us. His name is Thomas Pérez.” The warden smiled. “It’s a rather touching little story in its way. He and your mother had become almost inseparable. The other old people used to tease Pérez about having a fiancée. ‘When are you going to marry her?’ they’d ask. He’d turn it with a laugh. It was a standing joke, in fact. So, as you can guess, he feels very badly about your mother’s death. I thought I couldn’t decently refuse him permission to attend the funeral. But, on our medical officer’s advice, I forbade him to sit up beside the body last night.”
Not only does Perez sit up all night during the vigil, but he attempts to accompany the funeral procession during the heat of the Subsaharan climate and faints. Perez is a foil for Meursault, who falls asleep during the vigil and seems to be disinterested in the funeral (obviously showing signs of denial). Perez, however, goes out of his way, nearly dying, to honor Maman's death.
It is difficult to say who Maman was closer to, the unnamed woman or Perez, but it does rule out Meursault as an answer.