、。
A true gentleman wouldn't betray a friend.
Literal
True [possessive-の] gentleman if, friends [object-を] betray-[listing-たり] not-do would-probably.
本当の紳士なら ('a true gentleman would [...]') uses ~なら as an identity conditional. ~たりしない ('wouldn't do things like') with だろう ('probably') makes a hedged moralistic claim — not flat assertion, but a confident inference about character. 紳士 ('gentleman') is a borrowed concept from Western etiquette; in modern usage it survives mostly in formal or moralizing contexts.