There is no such thing as a 'joint international ticket'. Under the Warsaw and Montreal Conventions all passengers must have a ticket in their name. (Years ago there were domestic group tickets, but I think they have disappeared.)
Assuming there is nothing in the tariff requiring both passengers to travel (i.e. a family fare) there should be no reason the travelling pax would be refused transport. However at take first opportunity you should advise the airline that the other passenger isn't travelling. Do not simply show up and get on the plane - that will cause problems, tell them the passenger isn't travelling and to cancel all subsequent sectors.
Edited to add. Don't forget to ask for a refund. While the fare may be non-refundable any taxes are! This could be a significant amount.
Last edited by ExXB; 23rd May 2013 at 08:50.