Your post suggest you already know the best answer we can give; report this behavior to a judge as soon as you encounter the situation.
It is not your fault a spectator interfered, they should know better and they were in the wrong. However, perhaps in the future you could manage and hold your cards in such a way that it would be difficult for anyone but you to be aware of what you hold in your hand. This isn't always possible and it is not a solution, just a suggestion.
Sorry you had to deal with this troubling scenario. As a judge, I would make the offending spectator leave the area, possibly leave the venue. Penalties would be assessed if they were in the same tournament, of a variety of severities depending on the exacts of the infraction. As for your game state, there isn't much that can be done to help, it wasn't your or your opponent's fault. This is an issue that often has to be handled even at the highest levels of competition. It is difficult to fix.
Owner of Plus One Cards