Riddle me this

[dave01]

My mate has a Non Imm O A visa (multi) in his PP. His permission to stay stamp expires in Dec 2018. His visa dies on APR 9 2018.  So he is heading to Vietnam in the next couple of days to get a further 12 months on his return... ALL GOOD.  Where it gets interesting, is that he is planning on going home for a few months.. So, say the permission to stay stamp is APR 7 2019, is he able to return on THAT stamp with a re-entry permit, prior to APR 7 2019, or is the permission to stay cancelled when he leaves? 

I have no experience with this Visa.. I am concerned because he will have no valid visa in his PP on return.............. or does the permission to stay stamp live on? And he will be allowed re-entry  based on that stamp and a re-entry permit?  When I moved here, I came on a TV, converted to Non Imm and then was granted the retirement extension.. All pretty painless. So I did it a different way..

Any knowledge out there? @Edge ?