Author of the Antipodean Queen magical steampunk trilogy, and the Heest magical pirate trilogy for tweens (both published by OdysseyBooks.com), plus an increasing list of interactive fiction (listed at ifdb.tads.org).
Perhaps Roshan killed Naomi because he discovered that she and Elan were together. Naomi was very concerned about telling their friends, especially Roshan, because she was afraid of how they might react.
On the other hand, if Roshan had killed Naomi and taken her ring as a memento, it would be incredibly stupid to give it to Penny for her to draw!
It seems more likely that Elan killed Naomi - perhaps because she was still reluctant to tell the others about their relationship - and took back the ring he had given her. He might have returned to the house, and climbed into Naomi's room through the window Roshan had left open - or perhaps he never left after they argued earlier in the day after all!
Roshan and Elan shared a room, and Elan may have left the ring lying around. Roshan wouldn't have known that Elan gave it to Naomi, and he probably didn't realise that it was evidence which would incriminate her killer when he gave it to Penny.