Yes, user1 is committing a crime - but this is not surprising, almost everyone commits crimes every day.
What you are most likely talking about are "Lèse majesté" laws.
The question of jurisdiction is really a matter of "what does the jurisdiction in question say" and "how can they enforce this"? If you are not committing an offense in country B, it is unlikely country A will be able to extradite you, so unless you go back to country A they can't do much.
The worst Twitter can do is ban you if they are successfully pressured to by Country A or Country C.
Maybe anecdotally helpful - One of the places this is most strictly enforced (outside, perhaps, North Korea), is Thailand. It is noteworthy that a US diplomat was investigated for breaching this law, but as a diplomat he could not be punished - because he is not subject to Thai law, even in Thailand - "Thediplomat.com" reported : "Davies enjoys diplomatic immunity, so even if he is found guilty of lèse majesté, he will not be tried and punished under Thai law. However, Thai authorities have hinted that the investigation will continue and that Davies could be subject to some kind of reprimand from the government. Still, it remains unclear what such a reprimand would entail."