Hello Mortgage guy. Where can I find information on what qualifies as a 2nd home?

I am trying to close a 2nd home right now (I work for a mtg company in CA) and the borrower has a vacation home on the beach that is rented out seasonally. The lender wants to counter this to a non-owner. I am trying to fight this and your article caught my eye. Any guidance that you could offer would be fantastic! Thank you

placeholder text for bug in Chrome