I trust her to be discreet.
careful to use good judgment and/or trustworthy in various senses:
- trustworthy with secrets
- careful to avoid causing offense
- careful not to attract attention
- trusted to have good judgment
The form, discretion, is usually used as the noun sense of discreet, as in "I trust your discretion," but it can also refer to the ability or authority to decide, as in "The penalty is at the discretion of the judge," or "She abused her discretion."