A hen can raise guinea keets but I do not recommend it. The keets need a higher protein feed such as a good turkey or game bird starter for proper development.
The feed will not hurt the hen. The problem is the keets imprinting from the chicken. When the keets grow up they will not have the ability to understand that there is a difference between them and chickens.
Guineas have entirely different behaviors than do other fowl. When breeding season comes around and their racing and chasing along with attacks from behind with feather pulling and breaking are not understood by other fowl. They can cause great stress to chickens.
I have used broody chickens to hatch guinea eggs. Once the keets are dry and fluffy, I take them from the hens and raise them by themselves in a brooder.