I've been in private business for almost 40 years and licensed in NY for 20. I always use a CPA and he's good.
Pro-bono means no bill for services. Expenses are expenses but they must be reasonable.
The hours still have to be accounted for. I worked for 30 years keeping billable hours in 15-minute increments. I haven't used a CPA in a long time - but did, also for a long time.
Pro bono means for the public good.
Only had one non-paying client in all that time - a photog I was forced to bill an annual report through. He paid all but $300. I wrote it off.