Rohan to play second
Let's suppose that Rohan plays first and he picks 9. Then Karan will definitely pick 8. Now, Rohan will have to pick 7 or Karan will pick 7 in his turn. But if Rohan picks up 7, then he will score 16 that is beyond 15 and will lose. So one thing is for sure, no one will be willing to start with the highest digits.
Suppose Rohan plays first and picks up 1, Karan will pick 2. Then Rohan will pick 3 and Karan will pick 4. Now Rohan will be forced to pick 9. The score is 6 to 13 and thus Rohan will have no chance of winning.
If Rohan Picks 9 after Karan has picked up 2, then Karan will pick 8 and the score will become 10 to 10. Thus Rohan will pick 3 as picking 7 will send him past 15. Now Karan will pick 4 and Rohan has nothing to pick for winning. Thus Karan wins.
Therefore, you should suggest Rohan to play second.