It's quite true that you can never be too careful in choosing friends.

Literal

Friend [object-を] choose [purpose-のに] how-much careful-even overdoing [topic-は] not-exist, [quotative-という] [nominalizer-の] [topic-は] true is.

Wraps the familiar いくら~しすぎることはない proverb inside a ~というのは~である frame: 'the proposition that X is true.' A meta-commentary on the proverb, more formal and essayistic.