My friend had to replace the radiator in her Nissan Rogue and bought "blue" antifreeze as specified by her mechanic. After the job was done and she'd been driving for a few days, she went to return the extra blue antifreeze and discovered that it was meant for an Audi or Mercedes. Does she need to have her radiator drained and refilled or is it going to be ok?
There is no antifreeze "for Nissan" or "for Audi". There are liquids made by different manufacturers or under different trademarks that meet (or don't meet) certain standards.
The requirements for the cooling liquid are related to the materials used in the engine, radiators and other parts that the coolant runs in. And, rest assured, all car manufacturers use the same steel, cast iron, brass, aluminium and differnt kinds of rubber like anyone else. There ARE some subtle differences, of course, but they don't change the overall chemical properties of the material.
What's more, in a lot of cases, different car manufacturers source their radiators, rubber hoses, water pumps, etc... to the same pool of subcontractors, so the parts in different cars may happen to be made in the same place.
Usually, vendors boast compliance of certain vehicle manufacturers internal standards on the label. This doesn't mean that the substance is bad for some other car.
In some cases, the antifreeze (or the motor oil, for that matter) is branded with some car manufacturer brand (e.g. Audi ot Mercedes-Benz). This means nothing (except for some price tip).
Neither VAG group (=Audi) nor Mercedes-Benz, nor Nissan, nor Ford really produce antifreeze. And even if they do, they can't put some "other brand engine poison" in it. It is still OK to use it in any other car that requires the antifreeze to conform to the same standards.
If the antifreeze was specified only as "blue", this generally means "G11" antifreeze standard. This is somewhat old, but still pretty much widely used (even in new cars) type of antifreeze. Other types of antifreeze include "G12" (usually pink), "G12+", "G12++" (minor improvements over "G12", still pink), "G13" (purple), etc, etc...
Same color coolants are generally interchangeable and freely mixable.
Tell your friend to run her car happily and check for leaks under the car in the first month or two. An antifreeze change sometimes promotes cooling system leaks.