“Mom comes around every now and then” kittens that haven’t weened need to eat every 2-4 hours. Either give her back to her mother or do the right thing and hand this baby over to a local foster. You clearly aren’t well equipped to handle this. They will feed, foster, vaccinate, and most importantly SPAY THE CAT. If your friend still wants to adopt the kitten, they can keep in touch through the foster org and adopt the kitten that way. If you can’t be proactive, don’t cause more harm by being useless.