If a rare candy is given to level 100 what will happen
Since Generation VIII,
If a Rare Candy is used on a level 100 Pokémon that is able to evolve when leveling up (including those that evolve at a certain level, or with high friendship, or knowing a certain move, among other Evolution methods) and currently meets its Evolution requirements, this Pokémon will evolve without gaining a level; fainted Pokémon will not be revived if evolved this way. The player can cancel this Evolution by pressing the B Button, but this will waste the Rare Candy. If the player attempts to use a Rare Candy on a level 100 Pokémon that is currently holding an Everstone, the Evolution is not allowed and the Rare Candy is not consumed.
This does not work on Tandemaus specifically, as it must level up in battle to evolve.
Bulbapedia - Rare Candy
Nothing would happen unless the Pokemon is level 99 or lower. Since level 100 is the maximum level you can't make it higher even with the Lumoise Juice Shoppe you can't. Pretty much meaning nothing will happen.
Source:I tried it in my X game
Also you won't lose it.
You cannot give a level 100 Pokemon a rare candy. It will simply say 'It won't affect it' or something of that sort.
Answer: Nothing will happen, your rare candy will not be used.
Source: Experience
It won't have any effect.