Bottom line. Send the poacher back to Y. End of story.
FA: Please take your proper seat Mr. "Smith". FA: Poacher get your ... back to Y and take you assigned seat. FA: Can I get you something to drink, Mr. "Smith". Yes I'll have a G&T. Be right back with your pre-flt drink Mr. "Smith". Enjoy your travel on the "New UA" which I am proud to work for. After we reach a safe altitude, what would like me to do? Perhaps another drink, snacks or just a smile. I can provide all three on the new UA.
Mr. Smith is a happy camper and will stick with UA. THAT IS HOW IT IS HANDLED. At least it was until about 3 years ago on UA and still is on DL and AA.