My Queen "Phobos" has had difficulty having babies; she's had three litters totaling 12 kittens and only one made it ("Mazakeen," 8 months old) past a few days. Her 8 month old started nursing again after she lost her whole litter and she just started re-bonding. They weren't getting along for a while; after about four months the momma basically stopped being interested with her baby. Should I just let her continue? It breaks my heart she lost the babies. The vet told me to shoo her away, but it doesn't seem to be bothering the mom at all and they clean each other.
If the surviving offspring (technically it's too old to be called a "kitten") is female, I see no reason to separate them. Nursing can simply be comfortable and calming activity for cats - comparable to babies sucking on a pacifier or adults cuddling.
If the offspring is male, you should either (permanently) separate them or neuter him before he becomes sexually mature and inbreeds with his own mother.
You should check if the mother still produces milk. Since she lost her kittens, she should stop lactating soon, but if the other cat actually suckles on her, it might prolong that time. The production of milk requires much energy from the body and may cause malnutrition in the cat. If she still has milk and looses weight, a vet might help with either a vitamin shot or some medication that stops the milk production.