Best Method to Reduce Capital Gains Tax

[Jared Khan]

How does one go about lowering capital gains tax from the sale of an investment property?

Some context: a relative bought a duplex as an investment property back in 2012, and are planning to sell it.
They don't live in the U.S. and have no other income to report aside from the rental income from the duplex (as reported on their Schedule E each year).

The intention/goal is to maximize cash proceeds one way or another; no 1031 exchange as they're not looking to buy anything with the proceeds post-sale.

There's still an outstanding balance on the mortgage; no other debt involved otherwise.

Any advice on the matter would be greatly appreciated...thanks!