Hi Amber, and welcome! It is a real good idea to have a buddy (or 2) for the 'castaway' chick. They can share body heat to keep warm. They can learn communication from each other. If they are kept in another brooder or whatever in relatively close proximity to the rest of the chicks (within peeping distance), they will still communicate with them and the mom, which will help them socialize and become comfortable with one other. Also, they will make their own little peck order, even if just a pair. So, when you reintroduce them to the rest, they can already have their own little support flock. You could try rotating the other chicks for short intervals so they get used to the lone chick at least, even if the mom is not taking. That would probably also help when they get older. This way they will have imprinted each other.
Another idea, maybe try holding the mom and the chick together (maybe a friend can help if the mom is flighty) and speak in soothing tones, or play soothing soft music. They will associate the pleasant tones with you (assuming they are ok with you already) and then with each other, through conditioning. I don't know if there is anything on the forums, but it's worth a shot, a search on conditioning. I had a late chick in with my other chicks (no hen though) and while at first they were unsure who the new chick was, after giving each individual chick time alone with 'new chick,' they seemed to work things out to where now they are all comfortable. It did take a while though. Good luck!