Catalysis of the reaction: prostaglandin H2 + NADPH + H+ -> prostaglandin F2alpha + NADP+. This reaction is the reduction of prostaglandin H2 ((5Z,13E)-(15S)-9alpha,11alpha-Epidioxy-15-hydroxyprosta-5,13-dienoate) to prostaglandin F2alpha ((5Z,13E)-(15S)-9alpha,11alpha,15-Trihydroxyprosta-5,13-dienoate).