Help with 2 momma cats

My neighbor took in two momma cats who had 4 kittens each within 2 weeks of each other until she can have them spayed and find homes for everyone.

At this time the kittens are between 2 to 4 weeks old. The last 2 days both momma's want to haul one of the kittens (From youngest batch) off. My neighbor keeps putting it back to it's momma when the other one "steals" it.

Is something wrong with it that she may not be able to see? Seems to be healthy and normal. Should she keep placing it back, or remove it and finish weening it on her own, or let mother nature take course?

Your advise is needed

It really depends on the situation. Is there something that may be somewhat obvious that's wrong with the kitten? If that's the case, I would have her consult a vet. If there's something that can be fixed, they can help and if the kitten is suffering it can be put to sleep without suffering.
If not, I would have her try and seperate the 2 moms and litters. Keep them in seperate rooms where it's quiet and comfy. It may be a security issue.