Wait, lemme see if I got it right: She ment for him to clean it with a napkin, but seen him go straight for it she went along with it so he won't feel embarrassed?
Is Yamada the most perceptive character ever?
Has there been any signs of the guy she hangs at work with has any ill intent?
Other than yokoi, he seems to be the only other man she's been able to feel comfortable with, so if it turns out he's just trying to take advantage of her, she won't be able to trust anyone new she meets for a looong...