。
You must not be discourteous to your benefactor.
Literal
Benefactor [to-に] propriety [object-を] lack-[must-not-てはなりません] [gentle-よ].
恩人 ('benefactor, someone you owe a debt of gratitude to') is a culturally loaded word in Japan's obligation-heavy social framework. 礼節を欠く ('to lack propriety/courtesy') is a formal expression. ~てはなりません is the formal prohibition — the polite version of ~てはいけない. The よ at the end softens the admonishment.