Christian, the logging requirements are for the Safe Harbor only. Take a webinar or more on rentals. Soon. This could be a great niche for you.
Just like keeping a mileage log, we have to educate our clients into keeping "books" and tracking time. Some tradesmen are getting onboard with listing their hours on their invoices; they've been doing it all along when billing hourly labor charges. Otherwise, train your client to ask and have vendor write it in on the invoice. Or, make his own note "2.5 hours per Joe via tel" on the invoice.
Keeping all his vendor receipts is a great start. Have him hand out W-9s (mail them now for 2020 services already provided) and prepare the Forms 1099-MISC for him in January. I'm sure he knows his rental income and has the deposits to prove it and his notes in whatever form to track who's late.Give him a green ledger sheet for each rental for each year for tracking income and expenses.