Path validation helper in a 'path input box'
Start a Timer.

Whenever the text box changes, the user presses a key, the TextChanged event occurs, etc, start the timer. If it changes again, reset it. Create some timeout that, after which, the file validation would occur. (200ms would be plenty)

This has the obvious benefit of reducing the number of IO requests you do, and it helps keep responsiveness up. You can also make it asynchronous, and only hold the UI when doing actual updating.

As far as your code goes...

I would add braces in your else condition. I used to omit braces whenever possible, but I encountered more than one bug as a result, and it's just as easy to add them.

There's really not much else to comment on; there's not a lot going on here and it's quite clean to begin with.