Really no way to know for sure without the actual width the individual would take up in the seated position, everyone's body shape is different. I'd guess it might be close though.
Which Embraer aircraft is it? The widths vary among the Embraer models - EMB-170/175 (18.2"), E-145/135 (17.3"), or EMB-120 (17") [standard measurements from UA's website]?
UA's criteria should still apply, in theory, regardless of seat location - certainly the one-extender criterion would. Whether the FA would strictly enforce the armrest-down rule who knows. Not every seat has a moveable armrest.
Of course there is always some chance that the passenger could be moved to another seat for operational reasons (flight delay, cancelation, aicraft swap, etc) so it might be dicey if the only workable seat option is on the single-seat side and something happened.