housing both genders of betta splendens in the same tank is just asking for the death of at least one betta. bettas should only be housed in opposite genders when they are in the process of spawning, and the female is taken out right after she drops her eggs, to keep the male from potentially killing her.
in the wild, the male bettas stake out territories and generally don't leave those territories. the females roam about and spawn with the males, then leave after. the natural habitat of a betta is MUCH larger than a 55 gallon tank, and the females have the chance to get away from the males. the males' territories are spaced apart and males don't usually go into other male's territories.
i would suggest to return the male, or have him in a different suitable tank, and just keep the females in a sorority in the 55. as mentioned, you want to have enough females to spread any aggression out, so that one female does not get picked on (low lady on the totem pole). in a 55 you could easily do 5 females, plus other community fish that are considered non aggressive.