Our treasured Buff Orpington hen is not well, advice please

Hello,
My six-month old Buff Orpington, Percy, is not doing well, and it will break our hearts if she passes. I'm not sure what's going on with her, and I'm not having a lot of luck online finding a condition that fits her symptoms.

She just seems to be lethargic. In the morning, our other birds will be out in the yard, anxiously awaiting their breakfast, but she'll still be sleeping. In fact, we'll notice that she's in the coop a lot. She's always the last to get up and the first to go to bed. She'll go into the coop in the middle of the day and just sleep.

Her comb has sort of shriveled and is more of a dull grayish pink instead of a nice bright red, and she has noticeably lost some weight. She seems to be eating, but sort of half-heartedly and not as much as usual. She's shrinking before our eyes.

She has always been a cuddler. She seems to truly love people and is very affectionate. When we let them our birds out of their chicken yard to free-range, she will waddle over to where we are sitting and climb on our laps. She'll just sit there and let us pet her. But now I'm wondering if something has been wrong with her for a while now. When we open the coop to collect the eggs, she's usually in there and she'll sort of coo and wiggle when we pet her, but she is completely uninterested in whatever we're doing.

We did lose another hen about two months ago. That hen had some sort of infection that resulted in her abdomen splitting open. This doesn't seem to be the same thing, but I'm terrified the same thing will happen to our Percy.

I will say that I've heard some chicken sneezes and have dosed them with some apple cider vinegar in their water and I don't hear the sneezes anymore, but I wonder if it is Percy sneezing and if that has anything to do with it.

Any advice or information you have would be most appreciated. Thank you.

I would wonder about coccidiosis and maybe treat her.
Does her crop empty at night?
What does her poo look like?

I honestly had no idea what a crop was, so I just googled it. I'm not sure about hers, but I will check her in the morning and again in the evening. They're all asleep in the coop now. As far as the manure goes, it looks the same as everybody else's. No blood or diarrhea or anything. The rest of the birds are thriving. Out of 4 hens, I'm getting three eggs per day consistently.
I am a bit worried too about the eggs we're getting. If she is ill and she is actually laying, are the eggs safe to eat?

Have you wormed your chickens lately? I would worm her with Valbazen if she is starting to molt, but fenbendazole (SafeGuard Liquid Goat Wormer) is very good. Dosage is 1/2 ml of either, given orally, and repeat in 10 days. Treating her for coccidiosis with Corid or amprollium would not hurt her, if cocci could be the reason. Dosage is 2 tsp of the liquid or 1.5 tsp of the powder per gallon of water for 5-7 days, and I would treat all chickens. She is still at the age when they can commonly have coccidiosis. Symptoms are lethargy, standing puffed up, poor appetite, diarrhea with or without blood, and weakness.

She's only six months old, so nowhere near a good molt yet. Can I get those medicines you mentioned at the local tractor supply store?

Yes I re-read your post after I posted, and corrected mine. TSC may have Corid, but I would call all of your local feedstores in advance. It is in the cattle medicine section. Dosage is in my last corrected post. SafeGuard is in the goat section.

Yes! Tractor supply has stuff, treat quickly! If she doesn't drink the medicated water, take a tsp and keep dribbling on her beak and she'll start to lick it up.

I'm headed to get the medicines now. Thank you, everyone. I'll keep you posted. I will say that she was out of the coop this morning at feeding time, and she was pecking at the food. That's a good sign. At least she wasn't still sleeping.