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A true friend wouldn't betray you.
Literal
True [possessive-の] friend if you [object-を] betray-[listing-たり] not-do would-probably.
~なら ('if it's a true friend') is the topical conditional. 裏切ったりしない uses ~たり to present betrayal as one example of unthinkable actions — 'wouldn't do things like betray you.' だろう softens to probability.