The indicator should have a screw holding it to the subframe but, with a brand new bike assembled on a Monday or Friday (possibly the other three days too), it might have been left off. You will probably have to take the rear plate assembly apart to re-connect it permanently. This link
2018 Monster 1200 S will take you to a parts book for a 2018 Monster -- find "Diagram 28B, plate holder" to see how the indicators are assembled.
That said, you might be able to force it back into position temporarily. Use a lubricant that doesn't damage rubber; a small amount of liquid dish soap (Dawn or ?) mixed with water would be my first choice. Get the rubber support started in the hole, rotate it while pushing, use a small flat blade screwdriver to force remaining rubber through the hole. Be careful not to cut the rubber with the screwdriver. When the soap dries, it will stop being a lubricant and the indicator should remain in place...until your wife hits it again 😁
