A combination of mortgage notes for cash flow and desirable properties in high appreciation areas (not cheap, undesirable properties in the Midwest.)

I would recommend a combination of a house-hack and an investment property depending on your current cash position, short-term and long-term goals. 

If he has tons of time on his hands and wants to get his hands dirty, flipping could be an option as well, with a combination of his money + private money. 

When an entity is structured correctly you’ll be able to qualify for deep tax deductions against your w2 income through the business combination structure ( A structure that qualifies an individual for deep tax deductions) 

The modern features and amenities can attract renters, and energy-efficient designs might help keep utility bills down.

Be sure to clarify the lender's stance on total CLTV (Combined Loan to Value) at 100%, as not all will accept this structure.

Or a combination of both. 4 months on the market and lowering it to $1700 tells me that you might have a dump AND/OR your comps are WAY off.

Upgrading fixtures, adding modern tile, and improving lighting can make a big impact.

Combine this with poor ADU valuations (especially in single family zoned areas) result in hands off ADU additions costing significantly more than the value they add.  

That level of cash flow, combined with the appreciation potential, could definitely be attractive to a new investor, especially if they’re interested in STRs.