IND is one of the nicest/friendliest airports to opt-out. The TSA agents routinely compliment me on my friendly demeanor when requesting an opt-out and I routinely compliment them on their professionalism and general good mood. The agents at many airports make me feel like a criminal when I "respectfully decline" the nude-o-scope, at IND they chat and joke with me while they give me my massage.

Wait until she puts her bags on the belt, if they direct her to the full-body scanner that's the point to tell them that she declines. There's a chance they'll just send her through the metal detector up until that point so there's no need to opt out until then. In practice there are so few people and so many agents/lanes at IND that they almost never use the old-school metal detectors.

I'm also going to echo the other posters here, Japan is not rural Bangladesh, anything you need could almost certainly be replaced in <24 hours. No need to worry.