Is it a good idea to include my professor who left academia as a reference in my PhD application?

It seems unlikely that it would have any effect. I assume that he knows how to write a letter and explain things appropriately. The fact that he isn't an academic any more is less important than the relationship you once had and what he can still say about your prospects.

Of course, it is best if he has left a trail of accomplishments behind in his own work that will reflect on you. But he will likely know to point to that in his letter.

Don't depend too much on a single letter, however. You will want several if the institution will accept them.

People also retire, of course, and sometimes get asked for letters.