I'm not going to tell a secret that isn't my own.
Why aren't you going to tell the secret?
Because it isn't mine to tell.
It's very honorable to keep secrets that aren't your own. I'm sure the person whose secret it is will be very happy to know that you aren't going to tell anyone, and they'll see you as a very trustworthy person.
It's the right thing to do.
Doing the right thing can be its own reward at times. Trust is important so by doing this you'll be showing others that you are someone can be trusted.