Juzak Allah kheir
Praise be to Allaah.
It is not obligatory because no report to that effect has been narrated, so we refer to the principle that all things are permitted except those which have been expressly forbidden. As far as a mahram ( a relative whom one is permanently forbidden to marry this includes the mothers husband) for travelling is concerned, one of the conditions is that he should be a Muslim. And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid