Riot does penalize boosting if it involves account sharing. There is no penalty for playing with a friend who is significantly better than you and riding their skill up to a rank higher than your actual skill.
That said, remember -- even good players have bad days, off-roles, and champions they don't know well. Some of them play these things in ranked. It's entirely possible that Syndra WAS a good player just having a really terrible game, and normally climbs up pretty easily through the ranks.
Looking at that Syndra, it appears to be the case. He/She has a 56.2% winrate as Zed, with 274 games played at Plat 5. That's actually a pretty solid winrate over those games, and definitely explains the consistent climb, since his/her next-most played champion only has 13 total games this season.
He/She only has 8 ranked games on Syndra, and an extremely average 50% win rate. So it looks like you caught a Zed one-trick on an off game and an off champion. He/She probably is either trying to play Syndra -- his/her Syndra playrate has increased -- or picks it into champions he/she isn't as comfortable playing Zed into.
