A method shouldn't modify its arguments (shouldn't have side effects). Especially a getter-type method. You should probably rethink your design.
Your problem is actually related to prohpecy, so you might want to read its docs: https://github.com/phpspec/prophecy
Prophecy (and PhpSpac) make some things hard. In most cases it means that you're trying to take a wrong turn with your design.
You could try the following, but I don't think you'll get a reference to your variable:
$someService->isEntitledToDiscount($guest, $ticket, $discountAmount)
->will(function ($arguments) {
// do your evil
});
Recommended read: Command Query Separation.