。
She let the secret slip to her friend.
Literal
She [topic-は] that secret [object-を] friend [to-に] let-slip.
もらす ('to leak, let slip, let escape') covers everything from accidentally divulging information to letting out a sigh (ため息をもらす) to wetting oneself. With 秘密 ('secret'), it captures the unintended-or-careless quality of the leak — not a deliberate confession, but a moment where the speaker let something out they shouldn't have. に marks the recipient of the leaked information. The choice between もらす and the more deliberate 言う ('say') or 打ち明ける ('confide') tells you something about how the secret got out.