Coffee

Coffee
 

Could I use a? (rather than leaving it blank)

-She was drinking __ coffee.

Some people think coffee is a substance, then no a/an should be used. But my opinion is different: in modern English, very often in a restaurant we say a juice, a coffee, etc., to mean a cup of sth; I see no problem in the sentence above. Who is the correct one?

Both 'a coffee' and coffee without the article are correct.
You have understood the difference well.

Excellent answer. Thank you.