Just because he made a nest, doesn't mean he's instantly ready for breeding. They will often make nests when in a tank with still water, if anything to alleviate boredom or stake out their territory.
When breeding they need to have controlled introductions with them separate so they can't get to each other, at least for a week or two. Only when the male is posing at the female, that nest is built, and that the female is full of eggs, has vertical stripes and is being submissive towards the male can they be introduced. Some nipping and chasing is part of the natural courtship, but this can get out of hand. I had a male from a breeder, she gave up on him after he showed all the right signals, then killed the female, after the loss of a third female she gave up on him. Larger females than males can also kill the males - if she's larger than him they can't spawn anyway as he can't wrap her properly therefore cannot fertilise her eggs.
Breeding Bettas is not simple, and neither is raising the fry.