$500-600K/year in revenue.
Take away 1. app store fees (30%) 2. income taxes 3. hosting costs 4. contracting costs (he employs at least one part time engineer) 5. design costs 6. health insurance + other payroll taxes, and you are left with a small percentage of that as net income.
Now add in the hours of his life he has presumably put into this app for the last 8+ years. I'm going to go ahead and guess that it was more than 40 hr/5 day weeks
If you consider his skills as a premier iOS/full stack developer, simply getting a job in big tech would be a wayyyy better option for him financially. Heck he could pocket $500K/yr at Google/Meta in straight salary without any of the associated fuss.