"È davvero uno che lavora duro."

[arturotuono]

Would a subjunctive be needed here following "uno che"?

[marziotta]

No.

In this case "che" is relative. It could be translated as "who" as well.

Usually, if there can be a "congiuntivo" or subjunctive, in English there is a "that" that can be omitted.

I am sure (that) you are able to do it. - Sono sicuro che tu sia capace.

[undeadgoat]

No, because it's true that he works hard. If it were doubtful that he worked hard, we would need a subjunctive.