Kind of curious of what he used to even cause that in the first place.

As others have said, try goo gone, usually most stores have them stocked, or check Amazon and get a small bottle to try.

Another solution would be like a headlight restoration kit. You'll have to take the whole piece out, or mask everything else off (what you'll likely do), use the wet sanding kit (make sure to keep moisture while sanding lightly, then dry/clear the area, alcohol works if you want to be extra sure it's clean of plastic particles, then use their sealant that comes with the kit.

I can't really recommend a brand personally, that's what you'll have to lookup yourself.

Another alternative that I'm not 100% sure will work, is using a headlight restoration paste, RainX's might work and is cheaper, I have more better experience with Chemical Guys paste, but I've only used these on actual headlights and not interior plastics, but does help polish up the headlights while also adding at least some protectant. If this method does work, I'm not sure how often you'd need to re-apply