19-yr old collegiate swimmer developed dbl pneumonia in nov - still cannot compete/train at prior level. How long until he returns to prior ability?

Don't rush it. There is no definate time until full recovery. This varies from person to person, by how severe the pneumonia he had was and by the infecting organism. He should train as much as he can without becoming overly tired, doing less yardage and slower intervals. As he feels better he should gradually increase his workouts until he is back to baseline. If he overdoes it his recovery will be delayed.
Pneumonia. Not sure what you mean by "double pneumonia", but gradual improvement in physical capacity after major illnesses may take a long time. The question here is whether there is an underlying problem with immunity. This 19 year old needs a full and detailed evaluation, and if sexually active needs full testing for stds.