There is a lot I don't know about your specific situation, you weather, what the coop and run look like, flock make-up, such as that. I know that you are in a situation now that you have to deal with. It does you no good to tell you that I let a hen raise the chicks with the flock, not isolated to protect the chicks. When the hen weans them, they are already integrated. They still have pecking order issues but the basic integration is taken care of. I think it helps that I have plenty of room for them to interact.
I have had a hen wean her chicks at 3 weeks. The weather was fairly warm and they did fine. Don't get stressed out by that 90 to 95 for the first week and 5 degrees less each week after. Chicks don't really need that. When they are allowed to be exposed to the weather they acclimitize themselves. Unless you have really cold weather that chick should be fine on its own. You can provide heat if you want, being careful not to start a fire, but odds are it is unnecessary.
That hen has decided to wean the chick. She wants to abandon it. Her instincts tell her it's time. You could try letting the chick join the rest of the flock, but at that age difference and the flock not being used to it, that would be somewhat risky, especially if your space is tight. Besides it is an only chick and they are flock animals. When I have them, I have several to keep each other company. They tend to form their own separate flock until they are old enough to mix with the adults. That's normally around 20 weeks for mine.
It's a little hard to suggest what I would do in that specific situation, a lone chick not integrated, since I have not been there. I'd probably let the hen rejoin the flock and leave the chick in there by itself (though next to the flock) for a few more weeks. Maybe at 8 weeks try to let the chick in with the rest.
The hen is highly unlikely to keep it warm at night anyway, though you can leave her in there another night and see what happens at dark. I have had hens that abandons her chicks at night to roost with her buddies but hangs with the chicks during the day. I've also had hens that abandons her chicks during the day but sleeps with them at night. Most of the time when they wean them they abandon them totally, but there is a difference in most of the time and all the time.
Good luck! I can't give you any guarantees, just wish you luck.
