Often female cats tend to become motherly when there is a furry baby around. If she already has kittens, then it would be easy to get her to take a kitten into her crew. Fixed females may take longer to accept the kitten, but they are soon protecting the kitten as if it was their own. If the kitten is healthy, I would get it fixed quickly so that the hormonal changes that come with growing up will not cause your female to react differently to the kitten's scent. Also, try playing with the older cat and the kitten together to get them more accepting of each other. Also, pick up the kitten often to get it used to being held. Then the kitten won't be afraid and will be more comfortable when your best friend picks him up, which will stop the over-protectiveness of your other cat.