Gàosù tā  zhēnxiǎnɡ.

告诉他真相。

Tell him the truth.

Notes:

告诉(ɡàosù): to tell

他(tā): he,him

真相(zhēnxiànɡ): truth

Example:

A：告诉他真相。

     Gàosù tā  zhēnxiǎnɡ.

     Tell him the truth.

B：可以吗？

     Kěyǐ mɑ？

     Is that ok?