Would YOU do this? A tale of office drama.

Ok, so say you have a job where you serve as a basic assistant for multiple people, X, Y and Z. (so you are always very busy)

You have this co-worker who is supposed to be admin assistant for X, so technically speaking you work under X too, but also with this admin assistant who has been around for a long time:

This admin assistant co-worker is older and doesn’t have as many tech skills as you which means X relies on you even more than the admin assistant, but still cares for the admin assistant so won’t let them go.

This admin assistant:
-Has to be right all the time
-Never does their job and has you do all their work (they pretend they don’t know how to do something or just flat out give you their work)
-Takes about 2 days off a week
– Claims they mark “personal” on their time sheet for all the days they’ve “earned” but keeps it so secret (yes, the person processing the time sheet works in another building. This admin assistant literally has no one checking in on their time sheet)
-Boss X doesn’t care because he truly believes that this person has “earned” all the time off but really has no idea and doesn’t know that I am doing all of his tasks that he gives to my co-worker, the admin assistant.
– By the way- not one single person in the building likes this admin assistant.

How would you handle this type of situation? At first it didn’t bother me so much but after my co-worker totally burned me the other day for taking a personal religious holiday off and acting like they have authority over me, I am angry. I already have other work from my other supervisors and this admin assistant gives me all their work and watches TV on their computer in their office. I am so annoyed.
Would you sneak and look at the co-workers time sheet when they aren’t in the office? Would you do something else to seek revenge or personal gratification of knowing they are full of crap? Would you just turn your head and accept that this is the way of the world?
I find it hard to believe that this person can have so many days coming to them and lacks the skills to complete even the simplest of tasks. Seriously. It is a joke and I end up getting all the work.