Tell your opponent to get the rack and glue'em up tight.
You called the pocket for the '8', made it and the cue ball stays on the table- you win. The fact that the cue ball then knocked one of your opponent's balls in a pocket has nothing to do the the outcome.
In fact, even if you made your opponent's ball first, and then the '8' goes in behind it, you still win.
You win, cut and dried.

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