Hon hjälpte fattiga.

English Translation

She helped the poor.

Confused by how you can help an adjective? Should there not be a Definite noun here?

The noun (människor) is implied, but the “de fattiga” would be more generally usable. Maybe there is an idiom lurching here, concerning helping the poor. For other verbs I would definitely use de: “Jag älskar de fattiga och hatar de rika”.