That doesn't always work. Then what if the day of departure comes around, it's J0? Then he'll be stuck in Y class. He's not SE so he doesn't get first crack at any remaining seats at the airport.
He wants to know whether it's a wiser choice to just fly on UA so he can at least guarantee himself E+. I think it is as long as he can find a flight that has E+ (as some Europe flights on UA are operated by non E+ 777).