Is there a difference between 连朋友都没告诉过 and 即便是朋友也没有告诉过?
连朋友都没告诉过 = Had not told even his friends

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即便(是)朋友也没有告诉过(他们) =  Even though they (are) his friends, he had not told (them)

Notice:
即便 means &quot;even though&quot; or &quot;even if&quot;
There's a verb &quot;是&quot; (being) in &quot;是朋友&quot; (being friends)
More example:

连他父母也不知道他是同性恋 = Even his parents don't know he is gay
即便(是他父母)也不知道他是同性恋 = Even though (they are his parent), they don't know he is gay

Example of 即便:

即便(有)朋友也不能告诉(他们) = Even if/ even though he (has) friends, he can't tell (them)

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即便全国警察都来了也救不了死去的人质了 (Even if) the police of the whole country were here, the dead hostages could not be saved.