There are a couple of viruses than can cause the cat equivalent of a cold. Other than some supportive care, there's not a lot you can do other than to let it run its course. While I assume your indoor cats have been vaccinated against the viruses that can cause upper respiratory infections, if memory serves they do not provide complete protection, so be careful that you don't spread what the outside cat has to your indoor cats. Bear in mind, too, that once a cat had one of these viruses, they carry it for life. That means that they can still transmit it to others and are suspectible to getting ill again themselves.
If you notice the cat stops eating, you might try foods that are smelly (like tuna). Cats often lose their appetite when they can't smell their food. And if she really seems worse off, take her to the vet. In the very least, they can give her some fluids and/or an antibiotic to ward off secondary infections.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher