If Miss Kelly were our boss, we'd quit in fifteen minutes flat. That being said, we give Eilis props for having enough fortitude to work at her shop at all.
In many ways, Miss Kelly's unsavory attitude gives Eilis all the reason she needs to move to America. It quickly becomes clear to her that "Miss Kelly had a different tone" for each customer, greeting rich folks "warmly and by name" and saying "nothing" to others (1.8). This gives us a small glimpse of what life would be like for Eilis if she had remained in Ireland, especially when you contrast Miss Kelly's demeanor with that of the Bartocci's manager, Miss Fortini.
Because of all this, it's only fitting that Miss Kelly is the one to drive Eilis back to America. Staying true to her judgmental nature, Miss Kelly blackmails Eilis with her recently acquired knowledge that Eilis is married to Tony, which she presumably received from Mrs. Kehoe. This is super-lame, of course, but it might work out just the same. After all, if this is what Eilis will endure if she stays in Ireland, then she's got to get back to the States ASAP.