Well, all you know is that the droppers that person owns has 15 drops = 1 mL. I would suggest a little test that costs nothing except time (and a graduated cylinder if you don't have one): get a cup full of water and a graduated cylinder. Count out the drops it takes to get 5mL as measured by the graduated cylinder. Do this 5 times. Add the number of drops from each together and divide by 25 and you'll have a good average of # drops/mL for that dropper.
Let's see what I've got in the magic trash can for Mateo!