You can validate someone by demonstrating that you understand what they are saying.

Trumpophile: We are lucky to have the greatest president in the history of this great country!

Trumpophobe: You see Trump as a world-class leader, a truly historic figure.

In this little vignette the Trumpophobe has validated the Trumpophile, not by agreeing, but by demonstrating a fair understanding of what the Trumpophile said.

Simply paying attention to someone is a form of validating that person. So is showing that you have a fair understanding of what they are saying.There are other forms as well. In short, one can validate the person without agreeing with what they are saying.