This is how Lightning Speed finds out goes in Diamond Tiara's Bully Experience.
[Lightning Speed walks through Ponyville when he hears Diamond Tiara talking]
Diamond Tiara: Thanks again, Apple Bloom.
[He peaks out from behind a barrel to see Diamond Tiara and Apple Bloom]
Diamond Tiara: You're more than welcome to come over to my house.
Apple Bloom: Thanks, Diamond. See ya tomorrow.
[Lightning Speed gasps]
Lightning Speed: How can she be hanging with Blank Flanks?!
[He gets an idea]
Lightning Speed: That's it.
[He visits Zecora]