Poor little thing. It seemed healthy and slowly it started to look weaker and weaker. It's now lying down and very listless. The other peep seems very bothered. It's so hard not to be able to do anything. I don't think that I want to hatch peeps again. The whole process has been very depressing. Darned power outages!
If you can't get another chick or save the sick one, you'll have to be it's friend. You can give a mirror, but do tap on food and water often to make sure it eats. You can do a search on lone chicks or single chicks to find past threads on this topic.
I always handled my lone chicks a lot until i got other hatchlings. Seemed to work out pretty well. Best of luck.
I usually stand a feather duster up in the corner of the brooder so that the lone chick can snuggle under "mom". Once I put a little stuffed animal (a beany type parrot) in with the baby for company.