Also on the side of "a guest shouldn't be expected to inquire after a coaster."
I note that this was happy hour, so drinking was the point (sort of--I mean, she weasn't going to play video games and leave her drink to sit). And she set the drink down in order to have her hands free for adjusting a cushion--so she would probablyl have picked it right back up again, and she would have known that. (Host wouldn't, nec., but could have deduced perhaps.)
It takes a few moments for condensation to drip down the glass onto the table.
Also, it's possible (OP, do you know?) that is was a freshly made drink, and it hadn't accumulated that much condensation yet. The event was indoors, and if there was A/C, then there wouldn't have been tat much mositure in the air. Sure, any dirnk with ice should have a coasater, but for a drink that isn't going to sit for long, it's probably not that necessary.
So the guest may have been making rapid subconscious calculations that there wasn't going to be much moisture hitting the table in the time it took to adjust her cushion.