My friend's bike broke and this thing came out... From various sources I found that it's probably a cog from deraileur. He went to bike shops but none could find a replacement that's for this exact brand (and no one told him what brand it is even when he asked...). Therefore it would be great if anyone could give some pointers of what brand/size/etc so we could buy it off the internet (if you know where that would be really helpful too!)
I could also update with more info if needed, e.g. if you need the brand of the bike