Excerpt from Memory-Alpha:

Archer asks about the progress of Phlox's medical research. Phlox says that he thinks it is incorrect to be helping the Valakians – that the Menk are supposed to survive, and will only be allowed to properly evolve and prosper if they are not tied to the Valakians. Phlox explains that the "disease" is actually an inherent flaw in Valakian DNA; they've reached an evolutionary dead end. Archer demands a cure for the Valakians, saying he doesn't agree with Phlox's point of view. Phlox reveals that he already has a cure.

The next day, Archer enters the medical facilities. Phlox tries to again state that he does not believe that they should interfere with the natural pace of events on the planet. Archer cuts him off, and makes the statement that he has reconsidered the matter, and that he agrees that the Enterprise and the Human race did not go to the stars to play god for other species. They go down to the planet and give the Valakian doctors a medicine which will ease the symptoms and help them. They state they won't give them a warp drive, that their race must help itself.

Excerpt from Wikipedia:

Archer, meanwhile, is debating whether to provide the Valakians with Warp drive, ultimately deciding against it. Upon further investigation, Phlox learns that the Valakians suffer from the illness because their gene pool has reached a "dead end" and that the Menk are undergoing an "awakening process." He also finds that the Valakians have been stifling and underestimating the Menk. He has found a cure, but does not believe it would be ethical to administer. Archer considers how a "Prime Directive" would be helpful, and provides the Valakians with medicine that will diminish the symptoms for a decade, anticipating the Menks' natural evolution and new levels of understanding between them.

There were two humanoid species living on the planet Valakian with a pre-warp society, the Valakians and the Menk. The Valakians were technologically more advanced and regarded the Menk as less evolved, nothing more than hard workers. The Valakians provided the Menk with food and clothing, and housed them in compounds.

A genetic disorder, for which the Menks were immune, was slowly wiping the Valakians out. Archer was asked for help to find a cure. The Valakians also wanted warp drive technology to seek for help from others.

In the end, the Valakians didn't get the cure and no warp drive technology.

Were Archer and Dr Phlox right to deny the Valakians the cure?