Guy Friday is an idiom used to refer to a reliable and efficient male assistant who can assist with various tasks. However, there are several synonyms and alternative expressions for this term. For example, you can use the term "right-hand man" to describe a loyal and supportive assistant. Another alternative is "personal assistant" or "assistant," which indicates a professional hired to provide administrative or personal support to someone. Additionally, one can use "man Friday," which means the same thing as Guy Friday in a more formal and traditional sense. Ultimately, the term "Guy Friday" has several synonyms that one can use depending on the context and level of formality required.