。
My friend called me a coward.
Literal
Friend [topic-は] me [about-のことを] coward [quotative-だと] said.
のことを softens the objectification of 'me' as a topic of discussion — slightly less confrontational than 私を directly. 臆病者 (おくびょうもの) compounds 臆病 ('cowardice') with 者 ('person'), the standard suffix for labeling someone by a trait. だと is the quotative copula+と packaging the label as reported speech.