That type of lesion (inflammatory myofibroblastic tumor; usually benign and localized) is uncommon in general (< 1%) but when found, it is usually in the young (upwards of 80% in that population). Prognosis after complete resection is typically good, often excellent.
Nothing posted so far gives me reason to question the green-light for scuba. So I trust he's good to go. Does his exercise tolerance seem appropriate for diving? Does he have a feel for what is "normal" during and after exercise?
As an aside, pulmonary rehab sounds automatic but it isn't. AFAIK, there's yet no evidence-based standard for this in connection with lung resection like this; insurance approval is exceptional. Quite some years ago, there were several papers about exercise capacity after lobectomy which basically found that lung function improved noticably at 6 months as compared to at 3 mo. Further, leg discomfort was a more common complaint than shortness of breath; presumably from slow acid clearance due to loss of area for gas exchange. Serious complications (e.g. leaks, adhesions, torsion, nerve injury) usually show soon post-op and decline post-discharge; 6 months seems to be a common boundary in studies I've seen. But healing is an individual, evolving process. New exercises or levels of exercise could reveal new pains or sensations which may or may not resolve with time or repetition. Don't hesitate to ask his doctor for an opinion/evaluation if this happens. Best of luck.