"My taxpayer lended the money to his business partner to open a bar"

It's either:

My taxpayer lent money to his partnership, and the intent was to open and operate a bar. Now the partnership can't pay him back. In this example, he is part of the partnership (on the paperwork legally, and got a K-1 from the partnership's operations each year).

Or,

My taxpayer lent money to a person, and that person no longer has the ability to pay it back. In this example, he is only a lender and is not also a partner in the legal entity that owns/operates the bar.

You keep stating it incorrectly. Find out which way it is, in reality.

And it matters if any of this was paid previously or was no payment ever made?