It depends on the situation. A collapsed lung would need to be reinflated.
A bruised or injured lung may require oxygen therapy and you can read about that scrolling down on this site
http://www.petplace.com/cats/pulmonary-contusion-lung-bruising-in-cats/page1.aspx
Sometimes they can cope with an infection on their own.
Sometimes they can get by with a lower lung capacity and the damaged area just isn't used any more.
Best option is vet care and if cash flow is an issue here are some groups that might help you afford the vet bills:American Animal Hospital Associationhttp://www.aahahelpingpets.org/" Through the AAHA Helping Pets Fund, veterinary care is possible for sick or injured pets even if they have been abandoned or if their owner is experiencing financial hardship."Angels 4 Animalshttp://www.angels4animals.org/"Our services range from financial aid to complete treatmentto those pets and pet owners in need."Care Credithttp://www.carecredit.com/A credit card company for health care, including veterinary care."With a comprehensive range of plan options, fortreatment or procedure fees from $1 to over $25,000, we offer a planand a low monthly payment to fit comfortably into almost everybudget." They have a zero interest program if you pay within a certain time frame and they list the vets who work with their product.God's Creatures Ministryhttp://www.all-creatures.org/gcm/help-cf.html"This fund helps pay for veterinarian bills for those who need help."Help-A-Pethttp://www.help-a-pet.org/home.html http://www.help-a-pet.org/"Our efforts focus on serving the elderly, the disabled, and theworking poor."IMOMhttp://www.imom.org/"We are dedicated to insure that nocompanion animal has to be euthanized simply because their caretakeris financially challenged."The Pet Fundhttp://thepetfund.com/"The Pet Fund is a registered 501(c)3 nonprofit association thatprovides financial assistance to owners of domestic animals who needurgent veterinary care."United Animal Nationshttp://www.uan.org/lifeline/index.html"The m ission of LifeLine is to help homeless or recently rescuedanimals suffering from life-threatening conditions that requirespecific and immediate emergency veterinary care. We strive to serveGood Samaritans and rescue groups who take in sick or injuredanimals. In certain cases, LifeLine can also assist senior citizensand low-income families pay for immediate emergency veterinary care." They also keep a list of local and national help resources herehttp://www.uan.org/index.cfm?navid=163
Feline Veterinary Emergency Assistance (FVEAP)http://www.fveap.org/sys-tmpl/door/"Seniors, People with disabilities, People whohave lost their job, Good Samaritans who rescue a cat or kitten - any of these folks may need financial assistance to save a beloved companion."The Feline Veterinary Emergency Assistance Prog ram is a nonprofit 501(c)(3) organization that provides financial assistance to cat andkitten guardians who are unable to afford veterinary services to savetheir companions when life-threatening illness or injury strikes.
Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care