Tax Question- tax bill for seller

[Lisa Martin]

Asking for a friend!   ;-) 

Seller inherited paid off 8 unit apartment building in Southern CA when his mother passed away last year.  He wants to sell and likely won't 1031 exchange as he just wants to the cash.  I realize some tax burden is wiped upon passing of his mother but I'm not sure how much. 

How can he estimate any tax he will owe upon the sale?

Thanks for advice/insights!