You see an opening and press your attack.

The mare takes another step back, this one completely obvious. “I-I heard something and saw you three. I knew you weren’t supposed to be off by yourselves, so-“
“And how did you know that?“ Diamond Tiara steps forward, “Did somepony- say, maybe a princess- tell everypony they had to stay together?“
“Y-yes. We were-“
“So…“ Diamond Tiara interrupts and looks up at the ceiling, while slowly walking toward the pegasus, “What you’re saying is…“ She swings her head back down and stares the nervous mare down, “You deliberately disobeyed a princess.“
The mare recoils as her eyes go wide. Silver Spoon gasps, evidently having finally caught on, “That-That’s treason!“
“No!“ She argues back in a panic, “I wasn’t! I was worried and-and- oh Celestia!” She drops to her stomach and covers her head, “I don’t want to be banished and then thrown in a dungeon in the place that I’m banished to!“
Diamond Tiara lowers herself to maintain eye contact, “Well neither do we! So you keep our secret and we’ll keep yours!“

Seeing the cowering mare slowly nod in fear makes you feel bad. But you have to admit, it was effective. She says her name is Blossomforth. You aren’t sure what to tell her, though. If you just let her go, there’s no way she’ll really keep quiet.