If one mate has Fifa 14 and I have Fifa 15, can we verse each other on line? Or does Fifa have to be the same matching year to be able to play online together?
No, you can't play if you have different games. The games must be the same year to be able to play against each other online.
Has to be the same version.
This has to do with how the game synchronizes itself with the other player. It relies on the game engine behaving exactly the same on both machines (determinism). If determinism had been kept between the two, you'd have the same game, not an improved game.