Unfortunately the only thing you can do is to wait.
When a male is tied to a female it is because his 'knot' at the base of his penis has swollen and is essentially locked into the female to ensure a good breeding took place. As the swelling subsides, the male will eventually slip out of the female.
This can take anywhere from a few minutes to a half-hour or better.
DO NOT attempt to pull them apart as this can cause physical damage to the bitch.
I hope this helps.