Det can only be translated as "the" if it's followed by an adjective in the definite form (e-form) and a neutral-gender noun: det røde bord - the red table. Sometimes the noun can be implied: "Der er mange ord skrevet her, men det røde er vigtigt." - "There are many words written here, but the red one is important."
If the det is rather standing separately, it can either mean "it" or "that". "That" can also be more ephasisedly expressed with det der - that over there.
Or you could go for what makes grammatical sense in English. "She has done the very well" doesn't make a lot of sense (unless you take some grammatical liberties and are talking about a physical well, which would be translated as brønd in Danish). "She has done that/it very well" is better.