Without knowing the situation, it's reasonable this "non-friend" may have thought they were protecting the original poster. Assuming this person wasn't lying, there is no right answer here. Are you a better friend because you look out for your friend when they're not around and are willing to share a painful truth? Or are you the better friend because you protect your friend's happiness by not telling them?

I tend to take the former approach, but experience has taught the latter works better. Inflicting pain on a friend, even with their best interest at heart, is a good way to lose friends.