Given the amount of work that it takes to get one of these to market and to self manage, there just needs to be another compelling reason to buy. 

Offer to be the guy that restores the old family property to it's former glory and all....Still it'll be hard to not look greedy because you do want to make money after all.....main thing to me is being super PATIENT.

This means, on average, that this person is facing foreclosure or bankruptcy, and has no other reasonable options.

It's a big reason to think about estate planning, trusts, or just selling the house on your own. 

Also very good for many of the same reasons, but not as comprehensive. 

Also, when you’re ready to list, make sure your priced reasonably.

If I put myself in the shoes of the outside partner/investor friend, I would be very concerned because: 1) unclear and/or potentially conflicting business plan, 2) even with clarity in the BRRRR business plan, unclear in the viability of pulling it off with these market conditions, 3) the chances of dispute seems very high, especially if you're going to live in. 4 years ago, I went through a  similar thought process when purchasing an investing property with mainly my family money, while considering an outside investor to rehab for higher STR ROI... 

The main condition was I had to have the downpayment (20%) first, close the deal, and then execute the owner financed 2nd mortgage. 

If you trust your friend as a tenant, renting at $2,000 now and increasing to $2,100 next year is reasonable.

Stating that they are using the mortgage of the investment property as a reason because it would put me above their threshold.