How do I handle a "fake" co-author?
Hmmm. Socially:

If you have already agreed to it then you should just go along, graciously. In particular, don't take any action that makes vindictive people (who have power) want to sabotage your future. It is only one paper. There will be others. A paper is a good thing, even under such circumstances. If the advisor has some standing, that might actually be good for you. 

Ethically: 

Try to work to disassociate yourself from these people (leeches?) going forward. They are acting in their own interest, not yours. Advisors shouldn't play such a game. Find more honest and ethical collaborators.