The result of the play is a touchback. ALL kicks into R's (B's) endzone are touchbacks and because the returner did not possess the ball, the play remained as a kick. So even though the returner helped out the ball with the illegal kick, the result is still a touchback. (It seems like this should be a safety, but it's not.)
Now, how to assess the penalty.... Since this is a kickoff, the only option is to enforce 10 yards (yes, it's now 10 yards, no longer 15) from the previous spot and rekick.