A letter from a healthcare provider, therapists or social worker is the only documentation you can request. 

Attempt to find someone in your social sphere as the tenant ideally. 

that is 99% of the time a dead give away from a scammer.they are very good at what they do.If you go to your local meetups there will usually be real lenders there and or other investors that can refer real lenders.True private lenders dont troll social media looking for borrowers simply not done.

He wrote me in late October to say he'll be late with his rent because he is waiting for Social Security.

Each person present in the US will be given a tent as a means of social equity.

I don't have Uuuhhhs social security number but I do have his full name and birth date. 

Rehabs are taking longer because contractors are swamped and trying to socially distant, and the whole mess, and then lenders are also taking forever because they're so swamped with business because of the low interest rates, their heads are barely above water, etc.

Originally posted by @Ken Minard:Go over the tax implications with them.

Payments that you make normally fall into one of 3 buckets100% of the payment can be factored in somewhere on the returnPartial payment can be factored somewhere on the return0% of the payment can be factored in somewhere on the returnHouse-hacking also has considerable tax implications in the event that you want to sell this property.You can potentially defer a portion or all of the gain on the investment property with 1031 exclusion.You can potentially exclude a portion or all of the gain on the personal residence with section 121 exclusion

If you have to provide your social, they are going to hit your credit.