The phrase "working like a Turk" is a simile (it compares two things i.e., Amanda with a Turkish person "Turk") and it is also an idiomatic expression. An idiomatic expression is a phrase that becomes commonly used in a colloquial, or "folk" form, and it is based on the experiences of a particular group and their social circumstances.
This same expression can be found in a myriad of other varieties, depending on who says the expression, and where it is said. Expressions of this kind are often based on casual observations that different social groups make on each other, and the unofficial consensus that we, as humans, tend to reach on other people's lives in our consistent quest to understand one another.
The idea that Amanda "works like a Turk" contends that she is working very hard, and non stop. That could actually be taken as a compliment. However, there is a problem: we cannot assume that all people from Turkey are hard-working. In fact, we should never make any assumptions, whatsoever, of any race or ethnic group based merely on superficial observations.
This constitutes "labeling" and "profiling". Moreover, it could lead to other remarks that could mislead others and offend those who are being labeled.
As a person who is educated and self-aware, those phrases seem more ignorant than they are offensive. Really, how many people from Turkey can someone possibly know to claim that whoever is hard-working must, automatically, be associated with them?
Moreover, the fact that Turkish people are associated with hard work comes from a historical context in which they once were placed in a submissive position within society. For this reason, their years of hard work and servitude could be mocked by using this phrase.
However, Williams is presenting a realistic scenario using realistic vocabulary which, offensive or not, is spoken "at home". Is it offensive to the reader? It may be. However, Art is not used to moralize thoughts, nor criticize the colloquial expressions of anyone; art is an imitation of life. This is how Williams's choice of expressions should always be analyzed.