Treat them like an adjunct to the color guard. Find a spot to put them in the show with 5 or so yards space on every side of them, and let them do their thing.
Also: make it clear that the twirler DOES NOT PERFORM unless the routine has been passed off by you and by the twirler sponsor. If they say they're a twirler, and they can't twirl, they need to be in a uniform.
They need to perform the entire song's routine, beginning to end, with a minimum of mess-ups or improvisations. It needs to look like a coherent scripted thing, and not just like a kid out there tossing batons in the air at random.