、。
A true friend wouldn't say such a thing.
Literal
True [possessive-の] friend if, such thing [topic-は] say not would-probably.
本当の友達なら ('a true friend would [...]') uses ~なら as a hypothetical/identity conditional: 'if/when it comes to a true friend.' そんなこと ('such a thing') is dismissive — 'something like that' — and ~ないだろう hedges the prediction. The whole shape is a moralistic claim about loyalty cast as a soft conditional.