This will make it much easier to attain loans through an LLC or even personally once you have control over the units.

And to tag onto what Tom said...a property in DC owned by an LLC is automatically subject to rent control.

People buying STR are generally geared towards upper middle class who likely have careers where they are in some form of management and giving up control to someone else just isn’t in the blood, but objectively speaking, without emotion involved you really need to run your numbers. 

If you can't keep control of your personal finances, you are highly unlikely to succeed in real estate investing.

The exclusive option to buy gives the optionee a lot of control depending on the cost or consideration paid. 

If the owner files bankruptcy, goes into foreclosure or dies you'll have a difficult time getting your money back.I agree with @Justin Fox - it's best to have full control over the outcome of the deal.

I hate to do it but this is about damage control.

You should also know that any apartment building built before 1947 with 6+ units is subject to NYC rent controls, and tenants with subsidized rents are often hostile. 

I feel like that is control he doesn’t legally have. 

Granted this monthly number will likely be lower than his current rental income BUT the possibility is there to control the down payment and the interest rate, ideally netting him more through the life of the loan.