I've had a few friends catfished. One it was pretty obvious. He was stoked for us to meet her (he hadn't) and she was flying out. I cleared my schedule. About 3pm he called me in tears because she had to cancel. Me and my then-girlfriend came up to basically peel him off the walls for about eight hours.
I walked him slowly through all the warning signs that he'd been completely catfished. It was a good three, four-hour conversation. We left in the morning and he was in a better place. The next time I saw him he revealed all the stuff he'd dug up on her about her insane catfishing and evidence that he was hardly the first. He had a good laugh.
About four months later he said "You know, I've been meaning to ask but why, if you knew all along she wasn't real, you waited until I was in a crisis to tell me?"
I said "because once you've decided you're in love with someone the only thing you'll listen to is people telling you why you should be. It isn't until you're questioning that love that your friends can say anything. Otherwise we just have to sit by and hope it doesn't hurt too badly because otherwise we'll lose you as a friend."
He nodded thoughtfully. And still didn't ever let me in that closely ever again.