I believe the law he or she will have broken is in the same category as adding 'breach of the peace' to your charge sheet. I haven't seen the tweet but I don't believe it has racist or sectarian words, it's just a combination of words which in a particular context or situation will offend and could perhaps incite something more serious ie death threats or in extreme cases public disorder or riots etc. The tweeter is free to say what they like but they need to be educated in timing, tact, an when to go public and when to keep it for yer mates. Hopefully it will just be a copper round his house explaining the situation and giving him a caution 'for his own good' and then he gets a clip round the ear from his mum or dad. The media will be the main drivers of this one as without them the cops will just say 'stop being a wee dick as it gives us more paperwork