Ok first why do you think she retained her placenta? Second do you have any penicillin, if so give her 1 cc per 100 lbs in the neck or thigh. Make sure you pull the plunger out some to make sure there is no blood coming into the syringe if there is move the needle if you get penicillin in the vein she will die. I use 18 gauge needles 1 1/2 inches long. Horses do have discharge after foaling.
Until the vet has confirmed an infection, do not give the mare anything. There are two lives to consider at this point - mare and foal.