What's the outlook for a 16 yr old girl with 2 collapsed lungs, now has assisted breathing via machine?

Depends on cause: Lungs can be re-inflated & assisted ventilation can be withdrawn with recovery. The underlying cause of the incident is more important. Car crash: other injuries, brain trauma, period with poor oxygen to organs & brain. Asthma, cystic fibrosis, other lung disease. The human system can come back from major injuries as long as pre-injury health was good. Sickly people do worse.