Xerath is a completely different champion from Syndra and plays differently from her as well. (He is also the one who significantly outranges, not the other way around.)
If your friend is interested in Xerath, he should play Xerath. It's more important that he plays a champion he likes, so he enjoys the game and learns to play it. If he is interested in Xerath and (important, in my opinion) has enjoyed playing Xerath before, there is nothing wrong with him buying Xerath.
Picking the "correct" champion, the one that is strongest in this particular time frame, doesn't matter. It's not going to have an effect on him learning or handicap him in his Level 12 or whatever games and champion's strength changes in time anyway.
The more important thing is that he picks a champion he enjoys.