It's not suggested at all, actually. At any time the dominant female could turn on the lower-ranked female, and absolutely bully her to death. It's a ticking time bomb, one that's set to go off when absolutely no one is ready for it.
Male and female bettas should NEVER be kept together, EXCEPT for breeding. It's the same as the 2 females, eventually one will die or be very badly injured.
It's better to leave them either alone, or with a sorority of 4 or more, preferably 5 or 6. Even then, it MUST be in a 10 gallon or larger.
Sorry to be the bearer of bad news ):