The phrase "is acquainted with" refers to having knowledge or familiarity with someone or something. Some possible synonyms include "knows," "understands," "is aware of," "has experience with," "is familiar with," "has knowledge of," "is conversant with," "has a grasp of," "has been introduced to," and "is accustomed to." Depending on the context, other synonyms might be more appropriate, such as "is well-versed in," "has practiced," "has studied," or "has worked with." It's important to choose the right synonym based on the intended meaning and tone, as well as the audience and purpose of the writing.