Outsider: In the old days, there were no restaurants. Restaurants are a mid-19th century creation. Taverns in the past were similar to restaurants today. And until the 19th century, non-alcoholic drinks tended to be fatal (this is pre-pasteurization). Safe drinking water and soft drinks (which are made from water) only came about in the late 19th century or 20th century (and in some countries, have yet to occur). In the USA, drinks such as low alcohol beer and slightly alcoholic cider were common.
jewishfeminist02: A journalist taking a source to a bar to get them drunk, or to have a clandestine meeting, is NOT a reputable business meeting. A meeting typically involves multiple people, which requires a table, and once you have a table (as opposed to bar stools), it’s a restaurant, meaning your question becomes, how to act in a treiff (non-kosher) restaurant.