But what if you can't validate because what they are saying or doing is unhealthy for them and those around them and simply unrealistic and making you unhappy.
I think it is better to work with specifics than to generalize. I think it would help to think first - do not invalidate - and then - be validating.
One way to be validating is to be attentive and listen to what a person has to say. Ask questions and say enough so that they feel you really listened to what they had to say. Say you want to think about. Comeback with your thoughts.
Much for validating that "no that won't work".