It's hard to be sure without knowing his exact status when he was at the hospital the first time, but I don't think that is correct here.

The doctor examining him apparently was not told by the screening staff that Mr Duncan had just come from West Africa. If that info had been passed along, the doctor should have gotten a more detailed history. Mr Duncan had recently helped take the gravely ill daughter of his landlord in Monrovia to the hospital (a sad story in itself). The daughter died soon after.

With that history and the onset of his own symptoms, based on the CDC's protocol, I think immediate isolation would have been the prudent course of action. Severe disease comes on quickly once symptoms appear, and he was already starting to show symptoms and people become infectious when symptoms show, so I think a conditional release to send him home at that point would not be justifiable. Especially since he was staying with a family with children (not in the protocol, but we always try to protect children, right?)