What should you do if the female betta fish eats the male?

This would only happen if the male has died. The female could not have killed him so you must have done something very wrong. The dead fish should be removed from the tank and the water replaced befor it kills the femal too. Then if you decide to try keeping Bettas again, remember the basic rule that must be obeyed if you wish to be successfull keeping any kind of fish alive for any reasonable length of time. :- "1 inch of fish needs a minimum of 1 gallon of water, a cycled filter and at least 50% of its water changed weekly."