PRANK IDEA #1:
You know that friend you have? They’re not a good friend or anything but they’re a pretty good acquaintance and you like them more or less except they have a bad habit of getting WAY too drunk at parties and blacking out? Or maybe you don’t like them at all and think they’re really annoying. Either way works.
Anyways, next time you see them act totally normally but then when you get home send them (the earlier morning the email the better — 3 or 4 am being ideal) an email along the lines of:
“I just wanted to let you know that I really appreciate you opening up to me like that. No, nothing like that has ever happened to me exactly, but like i told you tonight, I do understand the feeling, and even though it took me a few years to get over I think you can come through this ok. I promise your secret is safe with me. Thank you for trusting me with this confidence.”
Then, except for occasionally flashing them a knowing smile, never mention it again.