Supervisor gets PhD student to do his entire job
It is really hard to judge a case like this. It could be anything from a wonderful academic training situation to extreme abuse. It would depend on a lot of things not stated here. What is the agreement between X and the professor, now and for the future? What does the professor actually do with the time freed by X? Is it to X's benefit in any way - say via joint research? Does the professor have such a stellar academic reputation that anyone standing in his/her aura is bound to be a success. Or, such a stellar reputation that they can get away with abusing students without anyone complaining? 

If the professor treats the student like a colleague and they have agreed between them that the extra work will put the student into an excellent position for the future, then all is well. If the situation is delaying the student in some way, or impeding his/her research or future then it should be condemned. 

But if you are to step into such a role, you need to make sure that you agree to it and accept whatever tradeoffs there are. If they are to your benefit, then it would be worth considering, though not necessarily accepting. 

You should have a conversation with X to see what the long term view is. You should have a conversation with the professor to set appropriate limits and expectations. 

One of the extremes, of course is Stephen Hawking. His students were willing to do everything for him. Happy to do it. Thrilled to be there. Even when it was hard.