Will adding anti-freeze at a stronger than 50/50 mix with water be harmful to the engine or other parts ofcar?

I just bought a used car, and after checking the strength of anti-freeze with tester it showed to be weak. My friend drained about half of radiator, and added about 3/4 gallon straight anti-freeze topping it off. He said it being a little strong wouldn't hurt, but after checking manual says should mix 50/50 with water.