Two issues I can think of...
1. Substance abuse. They will want to know that he's clean. That will require some evaluation and probably HIMS. I suspect that nobody would consider it plausible that a drug dealer would not also be a user.
2. Criminal activity. This is often considered evidence of mental health/personality disorders. Simple possession would not be an issue in this context, only #1 above would apply. But you cannot lie to the FAA, they will probably want the details of the conviction which is going to be more complicated since it involved drug dealing. So possibly an uphill battle there.
I suspect he can get a medical eventually if this is his only blackmark. A repeat history on either count would be very problematic.
The process might be smoother if he uses an aviation medical consultant before talking to the FAA, but I'm sure an AME could sort it out eventually.