Horizon Distance
There are several online calculators to give you this information. Just put "distance to horizon" or something similar into your search engine.
I input 300m and got a distance of just over 42 miles. I reckon that, in perfect visibility, you would be able to see Morocco from any point on the eastern Spanish coast up to just north of Estepona but not as far north as Marbella.
I hope that you win the argument.
SFS