Actually, "è tardi" is only used impersonally; a person "fa tardi" or "è in ritardo". Fare tardi refers to not being there on time, so once the person comes you must say "ha fatto tardi". To answer your question, I would say "Non è in ritardo", which could also apply to objects such as public transportation or scheduled events.
Would be nice if they could explain that in advance, instead of forcing the user to fail and look in the discussion for an answer.
4 years later, but yes, good point; I find the explanatory notes insufficient. Subjective, I know, but that's what these chats thrive on - subjectivity.
Tardi vs In ritardo:
In Italian there is a pretty strong difference if you're talking about time itself or about someone/something being late:
• It's late (it's a late hour): è tardi.
• It's late (the train is): è in ritardo / fa tardi (sta facendo tardi).
As you see, "being late" is translated with two different verbs depending on which you choose; there's usually no difference with other verbs.
e.g., "he arrived late" is "è arrivato tardi" or "è arrivato in ritardo".
The word "ritardo" means "delay", and the expression "essere in ritardo" means "to be late", referred to a specific person or object:
• "Scusami se sono in ritardo." ("I'm sorry I'm late")
• "L'autobus è sempre in ritardo." ("The bus is always late").
The adverb "tardi" means "late" as well, and used with the verb essere could mean "to be late" as well, but it's only used in impersonal expressions:
• "È tardi!" ("It's late")
• "Sebbene fosse tardi, sono riuscito a fare la spesa."
("Although it was late, I was able to buy groceries").
The expression "tardi" and "in ritardo" can be exchanged without any problem when they refer to a regular verb:
• "Sono arrivato tardi." / "Sono arrivato in ritardo." ("I arrived late")