Decent dhoti is fine and will be accepted, offcourse check with airlines. But, if he is traveling abroad first time whats wrong in trying trousers first time. You are deliberately making a story out of it. He is watching all modern things on TV and seeing it around, then why the hitch of not changing himself.
This is a stupid discussion, if he does not want to change his mind as 90% people dont do it easily so let him come in the dhoti.