Respiratory issues are very common in rats and, being prey animals, they hide illness well. Most rats will develop respiratory issues at some point in their lives.
Find out if they have an emergency appointment, sometimes clinics leave a spot or two open and don't fill them until the day of. It's important to call first thing to try to get those slots.
If you have a Ventolin (arbuterol) inhaler it may help with her breathing but it won't be a huge improvement. Don't bother with the Benadyl, it will not help and you will only stress her giving it to her. If you get meds for her, DO NOT force medicate her in that state. Find something to mix her meds with that she will take. I use mashed banana and PB with a bit of coconut oil and strawberry syrup and that seems to work well here.
If you have baby food, offer her some or some blocks soaked in warm broth. Baby cereal or Ensure are the types of foods that encourage mucus so she's unlikely to take those. She's not eating because it's too hard to breathe. That's why her mouth is open also.
A cool mist humidifier may help but, again, it will only help a small amount if at all. Keep her cage very clean - the more ammonia you can remove from her environment, the less she will strain to breath. (Use dish soap and water or something other than vinegar or strongly scented cleaners to clean the cage.)
Don't separate her from her friends unless she is being bullied. Even if the vet tells you to. All the rats in your household are already exposed. Rats are exposed to many pathogens throughout their lives but often they are not affected by them. It is not unusual to have one or two rats in a colony get very ill and the others be unaffected. Rats are different than other animals in this regard and vets often give poor advice as a result. Separating her will only cause her stress and make it harder for her to get better.
If you can't find a vet to see her tomorrow, you may consider euthanasia. Most vets would recommend it in the state she's in (if I'm right, which I may not be) but without aggressive treatment, she really has no chance and she will suffer.