Every club seems to have at least one bar lawyer who will protest the slightest infringement - and you know how popular that person can be!!!
Good answers above ^, if it's just club racing a chat afterward does no harm - BUT, if it becomes a regular thing and that person is using the friendly approach to their advantage then they have to be protested to stop the rot. (We've had exactly that situation in our club a few years back where one particular person seemed to decide that rules needn't necessarily apply in club races and it came to a head with one apparently deliberate port/starboard that could have ended badly.....). It's a difficult one to call, but if it's within your own club then you usually know the character of the participants.