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Sir, may I have a cold drink? No lemon, please.

 “Sir, may I have a cold drink? No lemon, please.” 
A kosher consumer walks into a hotel lobby, and orders a cold, non-alcoholic, kosher drink at the non-kosher bar. Though the glass used to serve the drink may be treif,the non-kosher status of the glass does not transfer to the drink since there is nothing hot (such as coffee or tea) or sharp (such as acidic foods or strong drink) currently being served. Though halachah normally restricts the use of non-kosher utensils so that a person should not mistakenly use them in a prohibited way, this concern is negated by both the provisional and non-Jewish character of the setting. Of course, the customer should ascertain that the drink is indeed kosher.
What if a slice of lemon is tendered with the drink? A lemon is a davar charif (a sharp food), and is therefore subject to stricter handling in a kashrus situation. The knife the bartender uses to cut the lemon may have previously been used on non-kosher meat, for example. This may render the lemon treif—even if the knife is perfectly clean. In this situation, the customer should remove the slice of lemon, but may thereafter partake of the drink.
If the drink is served from a designated bar that serves nothing but beverages, it is assumed that the knife was used to cut only fruits and vegetables for drinks, and the customer may consider the slice of lemon kosher.

https://halacha2go.com?number=599

Practical Halacha: One minute a day. By Horav Yosef Yeshaya Braun, shlita, Mara D'asra and member of the Badatz of Crown Heights.