Can a beneficiary challenge a South Dakota trust's "no contest clause"?

Yes.  In South Dakota, a "no contest clause" can be challenged on the grounds of:

 

             (1)      Fraud;
             (2)      Duress;
             (3)      Revocation;
             (4)      Lack of contractual capacity;
             (5)      Undue influence;
             (6)      Mistake;
             (7)      Forgery; or
             (8)      Irregularity in the execution of the trust document.