I would argue that the reason why Leona (as well as Blitzcrank) is not technically in the "Support" category is because their stats have them more considered to be tanks. Yes, Taric is also a tank, but he has abilities that directly influence his allies -- unlike Leona or Blitz which only has self-oriented abilities (though yes, they can be used to help allies, but so can almost every other champion).
As for it messing with your friend's ability to get the level 7 mastery, I don't think you understand how the ranking system works. The simple explanation is that it compares your friend's in-game statistics with others that have similar MMR. This is why it is actually easier to get S's on support champs when you successfully play them as non-support because your stats end up higher than others. Basically, what I'm trying to say is that it does not matter what class Leona is in. What does matter is how other people play her.