5 week old Kitten question?

A family friend took a kitten away from it's mother WAY too early, at 5 weeks old, because he was impatient and thought he could take care of it now. He is however, not taking care her at all, and I am afraid for the kittens life. I have another friend who's cat has kittens who are also 5 weeks old, and I was wondering if it would be a good idea to introduce this kitten with these kittens. Will the mom possibly take care of her? Or will she abandon all of her kittens? I just want some opinions. I do plan on getting kitten milk for her and teaching them how to stimulate the poop, but she needs a mom.