Uploading a preprint with wrong proofs
Your collaborator is a jerk.  Don't listen to his nonsense about you being naive; that sounds more like he's trying to justify his bad behavior to himself.  You are quite right that this is not appropriate professional behavior.

However, I don't think it rises to the sort of ethics violation that could threaten a career.  Now that you have a proof of the results and (presumably) have updated the arXiv preprint, the scientific record has been corrected, so I think you're back on firm ground.   Besides that, I'd recommend:


Don't do it again.
Don't work with that collaborator any more.
When you mention this result in your future work, always cite the other group together with your own.  Don't bring up the issue of who was "first" - even if you had been legitimately first, it just seems like bad manners to brag about it, as your collaborator seems to be doing.  
If you see other authors citing your paper, contact them to suggest they cite the other group as well.


I don't think it is worth bringing up to an ethics committee or trying to "whistle-blow".  I'd just treat it as a learning experience: there are jerks in the world and you don't want to be like them.