You quarantine because of possible diseases or parasites. Some parasites you can see like mites or lice. Get a fecal count at the vet if they will do it to check the poop for worms or Coccdiosis. You are not likely to see anything for Coccidiosis and probably not for worms.
Diseases and parasites can be transmitted by them eating and drinking from the same bowls, pecking in ground where the others have pooped, transmitted by various insects like mosquitoes, grasshoppers, or grubs, or spread on the wind. The further apart you can isolate them the more things you are protecting against. The general recommendation is a 4 week quarantine.
Some flocks have natural immunities. They may have certain parasites or diseases but have developed immunities to the effects. They can transmit those parasites or diseases but never show symptoms. That hen is probably at as much risk from your flock as your flock is from her. Quarantine is not much help against flock immunities. If you want you could isolate one of your current flock with her and see which one gets sick if either does.
To me that looks like a battery hen, a commercial egg laying hybrid, that probably escaped while being transported. She could be a pullet just getting ready to lay but my guess would be a hen that has finished a laying cycle. As such she would almost certainly molt and then return to laying like gangbusters.
I understand many people on here think the commercial operations are disease infected pits of vile. They actually are much more biosecure than the vast majority of us are, their livelihood depends on it. She may have picked up something in her travel so she is still a threat. Even if she is someone's lost pet she is still a biosecurity threat.
I take biosecurity seriously. I don't add any chickens to my flock unless I hatch the eggs or get the chicks from a reliable source. Many people add chickens regularly and don't have any problems more threatening than a parasite that can be treated. Some people show chickens which means they mingle their chickens with others. Those typically require a medical certificate that they have been inspected but those inspections don't necessarily catch everything. Your highest risks are probably going to be something that is more inconvenient than dangerous but some people have lost their entire flocks in situations like that.
I can't tell you what to do, that has to be your decision. I sure cannot give you any guarantees one way or another.
