Let them know that you are there to gather information and to help them find answers and solutions.I would recommend books on persuasion and understanding human nature.

sending eviction-type letters can be persuasive enough to scare the tenant into paying the full amount, but try a friendly conversation first.

Or maybe you are just more persuasive!  

Be honest (and persuasive) with your intentions: you want to win in this deal, but you also want him to win. 

Introduction to Negotiation: A Strategic Playbook for Becoming a Principled and Persuasive Negotiator from Yale University. https://www.coursera.org/learn/negotiationI recently started taking this class and have found it very interesting so far.

(Also, not sure if my wife is on board living in an old building while we fix it up..haha that might take a lot more persuasion)

I'm also a fan of "Influence: The Psychology of Persuasion."

Entertaining and informative reads with great insights and highly persuasive arguments. 

As a executive in my previous career, my experience was that broad diversity of background among my staff--political persuasion, age, race, gender, experience, professional background--lead to richer discussions, more professional disagreement and thus, ultimately, better decisions and better outcomes. 

With limited comps for a 1926 SF 2/1 property which which needs a full gut rehab (has never been updated), I am trying to determine how to make a persuasive offer to the seller/family friend before she decides to put the property on the market in 2-3 weeks.