I doubt it will hurt the game as much as most seem to think. Far Cry has an own big fanbase and it's a first person shooter, they won't just stop playing Far Cry because of RDR2 releasing weeks/months later.
If it would be a new franchise, ok, but Far Cry can stand for itself as long as it is a good game.
It might get some less sales but I think it will still do pretty fine.