A close friend huh?
Question # 18 on the Airman Medical Form 8500-8 states, "Have you ever in your life been diagnosed with, had or do you presently have any of the following: f. Asthama or lung disease". I hope he didn't check mark 'No'.
He also signed the bottom which states, "All statements and or answers provided by me on this application form are complete and true to the best of my knowledge..."
If he flies alone and has a asthma attack, he might end up crashing the aircraft in a school or a mall and kill tens or hundreds of people.
My suggestion - tell him to see a doctor, get a full medical history and report it on the next medical.