Budgie nostril clogged up

Hi folks
My mother in law has a grey budgie. One of his nostrils seems to be clogged up - he is fine in himself, eating, chatting away as normal and the other nostril is perfectly clear. He has had this for quite a while now and someone told her it will kill him so she has now started to worry.... Any ideas please?
She is nervous about taking him to the vet in case the stress kills him.