Then I had an uber liberal Jewish friend who was RIDICULOUSLY Kosher. He would avoid any dairy item near a meat item on the buffet. Wasn't even in the same item! Just proximity! He freaked out when he accidentally took Chicken Cordon Bleu.
Kosher food can only be prepared in a Kosher kitchen. Separate sets of pans and utensils have to be maintained for meat and for dairy, and any items meant for use on one must never touch the other. Meat and dairy cannot be eaten in the same meal. And if the kitchen is used to prepare anything that is not Kosher, then it's no longer a Kosher kitchen, and nothing prepared there is Kosher.
A strictly-observant Jew could not eat food from a restaurant that was not specifically Kosher.
The meat/dairy thing is one of many examples where the Jews started with a law that God gave them, and then, fearful of accidentally violating that law, created layer upon layer of increasingly-strict “hedge laws” to keep them safely away from any risk of accidentally violating the original law, and then more “hedge laws” to keep them from accidentally violating the previously-established “hedge laws”, and so on. Thus, they got from “
Thou shalt not seethe a kid in his mother’s milk.” to making sure that they never ate any meat and any dairy together, and that they never prepared or ate meat using any pans, dishes or utensils that had ever touched dairy, and never prepared or ate dairy with any pans, dishes, or utensils that had ever touched meat.
As for myself, my Mormon religion prohibits the consumption of alcohol, tobacco, coffee, and tea; and although I cannot honestly claim, at this time to be a fully-practicing Mormon, I do strictly avoid these things. I must, however, admit to a caffeine addiction. The use of caffeine is significantly-discouraged by my religion, but not outright prohibited as long as it isn't in the form of coffee or tea.