Is there ever a time when it would be better to not tell someone they are being catfished?

I always had my suspicions about a certain person online and about a month ago a friend of mine came to me with evidence that this person was not who they claimed to be.

I knew someone who adored this person. She was experiencing some difficulties and relied on this friendship. I decided to not tell this acquaintance what I found out because I knew it would devastate her and make her more miserable than she already was.

I knew there was no chance that the two would ever meet so I never confronted her. Was this the right choice?

Is there ever a time when not telling is the best thing to do?