If you're truly doing IF, you shouldn't add either to your coffee.

Martin Berkhan, the guy that popularized the 16:8 "Leangains" approach puts milk in his coffee. From [0], "So, it’s a question of a dose-response effect. Can you have some milk in your coffee? Sure, I wouldn’t worry about it and I have it myself. Life would just be too damn boring with only black coffee, especially if you’re used to having some milk with it. How much milk/cream? I would put the limit at 50 kcal total used throughout the fast. That’s about 1 deciliter or 1/2 cup 2% milk."

https://leangains.com/questions-answers-5/

None. I drink a cappuccino after lunch.

Capuccino has milk in it.

Yes, that's why I wrote that drink it after lunch, when I've already had calories and therefore am no longer in the fasting period.

I'm sure they're aware. After lunch is when the fast breaks, so they are free to put whatever they like in their coffee.