Your daughter probably lost her Dutch nationality on 1 April 2013, 10 years after the dual nationality rule change in 2003. The rule that went into effect then is that a Dutch citizen who has other nationality and lives outside the kingdom or the EU for 10 years loses Dutch nationality. If that is true, it would explain the failure of her earlier passport application.
She might not have lost her Dutch nationality, because the 10-year period can be reset by the issue of a passport, ID card, or official declaration of Dutch nationality. There are other exceptions as well; see RWN art. 15.
If she did lose her Dutch nationalty, however, then your daughter would have to migrate to the Netherlands with a residence permit rather than a passport. Once she's in, however, she might be able to apply for Dutch nationality after only one year through the option procedure.
Unfortunately, it does not appear that any family-based immigration route allows one to sponsor children over the age of 18. There is, however, a return to the Netherlands route for former Dutch citizens, but it seems that she cannot use this if she lives in South Africa, unless she was born in the Netherlands. She might also have trouble with the "special ties" provisions.
Her best bet might be to try to get a work visa through an employer or to set herself up as an entrepreneur and get a work visa that way.