How do you report rent to own Income?

Sounds like a land contract to me also, if it is then your client sold his rental property on the installment method. see this link to get you started: https://www.irs.gov/taxtopics/tc705

Depends on the facts and circumstances. 

https://www.ccim.com/cire-magazine/articles/lease-option-or-installment-sale/

I don't think the term "land contract" is even used in most states.  The concept is always available, though, under different names.  But the "rent to own" schemes I have seen are usually just rentals, with enough of an option for the tenant to buy that it will keep the place from sitting vacant.  Once the tenant moves in and finds all the flaws, they decide to look elsewhere -- the grass is always greener, in some other yard.  These days I'm surprised that a landlord would consider offering a purchase option, because tenants are already lined up (in most places) looking for a place to live, and the real estate market doesn't show much sign of cooling off.  

Anyway, don't get hurt from others taking a stab in the dark on telling you what this is.