When is it permissible for a mother whose husband has recently passed away to travel to reside with her son in another country?

A widow must observe the 'iddah (waiting period) of death in the house where her husband passed away. She is not permitted to travel unless it is absolutely necessary. The proof for this is what At-Tirmidhi narrated from Furay'ah bint Malik, that the Prophet (peace be upon him) said to her: "Stay in your house where the news of your husband's death reached you until the prescribed period reaches its end." This is the view of the majority of jurists and the Companions.