I have this user who wants to see result R2.
I tell them process P1 (which they are doing) will never give result R2, it gives result R1.
If you must see result R2, do process P2.
So they do process P2, then process P1 and expect to see the result of R2 for process P1.

How else can I explain that by doing A they will never get B, but at the same time explain the valid way of getting B?
Tell them P1 is like throwing mud on your car, P2 is like washing your car, R1 is a dirty car, and R2 is a clean one.
Throwing mud on your car will never make it clean; it will only make it dirtier. If you wash your car, it will get clean.
But if you wash it, and *then *throw mud on it, you will still wind up with a dirty car. It doesn’t matter that you washed it first; you also threw mud on it after that.
So you must wash it, and then not throw mud on it, and you will wind up with a clean car.
If you want R1 do P1
If you want R2 do P2
Have them decide what they want, then tell them how to get it, rather than wording it in the sense that “doing this will give you that”. Some people’s brains really can’t understand it when expressed one way, but totally get it when said the other way.