I certainly hope you will contact United about this.

I know most people won't bother since they were not directly effected, and I can't fault you if you don't... but this kind of attitude is insane.

I suspect that since the person wound up in your original seat, you could ID that passenger by the seat number. From there, UA could pull up the record locator and the actions done @ the gate and realize that a paid C/J fare passenger wound up in Y.

If UA is curious to know why people are choosing to fly other airlines, this incident would be a perfect example. Plus, I suspect that the affected passenger will probably write a letter, and your's will certainly corroborate his experience.