L'Oreal Color Zap or Color Oops? (Special Case)

Have you not considered anything more temporary?

A wig? A hairpiece to cover the fringe/bangs? A temporary black spray that washes out?

I don't really understand your plan. At what point in this are you dying black? What are you planning to use to dye it black? This can affect what you'll use to remove it. Sometimes it's best not to have a plan as removing colour doesn't always go as planned.

As for what remover? Always the less harsh option. I'm in the UK so I've used neither of the options, but we have similar products here.you can use colour removers up to three times, so it's always better to do that than use peroxide or a harsher type of colour remover. So definitely color oops. You then need to leave it at least a week before you use anything with peroxide.

If I have understood what you're saying correctly, this is all after you've dyed the hair black? If this is the case, the good news is, the sooner you use a colour remover after dying, the better it works. So dye your hair black as close to the wedding as you can and remove as soon after the wedding as possible and it will most likely be fine. It's not that likely you'll need the hicolor hiloghts anyway. The usual issue with colour removers not working is that there's several layers of dye build up and it's stained the hair. Though again, it can depend on what dye you use, some are far worse than others.

I am confused too, I thought she meant she only wanted to remove the purple, so all the hair is black?
In which case I have no idea if gentian violet will come out with a stripper (Colour Zap) or a colour remover (Color Oops) but my guess is neither. It may respond to fading methods, but if black is the goal, just dye over with a black semi.
True semis/direct dyes (such as the brands sold here) do not damage hair, there is no need to make your own colourants to avoid damage. (This also confused me, you use gentian violet to avoid damage, but are willing to use Hi-colour Highlights, possibly the most damaging product on the market?)
Then you can fade the black out after the wedding.
Colour removers such as Color Oops are designed for removing oxidative dyes ie permanent or demi permanent dyes that require developer (peroxide) They can work on direct dyes (Manic Panic etc) but the results aren't guaranteed. they are  not damaging, but can be extremely drying, which can damage already fragile hair.
Colour strippers (such as Color Zap) are basically weak bleach, definitely damaging, and not always effective as they can drive direct dyes further into the hair shaft making it harder to remove

I read it as, she's going to dye it black (over purple?) then strip it to go back to go purple as its just temporary. But that's why I was asking what black dye she would be using.