請教英文冠詞使用

You are correct that if a 改為 the means pointing at a specific friend. In this sentence, a is correct as you are describing a general point of view.

We use a when we describe something in general and we use the to describe something specific.

Example for using a, a car (the correct word should be vehicle) is usually more expansive than a motorcycle.
Example for using the, the car in the showroom is more expansive than a motorcycle.

Note: Please note that 卑劣的行為 is depicable behaviour. So, correct sentence should be written as
To betray a friend is a depicable behaviour.

Hope this helps.

Francis from Canada

原句只有這一種寫法，甚麼也不能改。我試按你問的原因解釋一下！

因為這是一句真理，這裏第一個a的意思不是指一個朋友，而是任何朋友(any one friend)，所以是單數，冠以a。第二個a base的a才是量數詞一種行為的a。字是相同的用意不同。

希望你看得懂。

all perfect sentence, no need to change