Taking a sohc head to change to a dohc setup (keep the same head, adding an other cam, customising everything...) would be almost pure fiction but i guess i you give 1million dollar to a team of expert... they would be able to do it.
But if your friend is talking about a sohc head swap to dohc... yes you can
Exemple, take a f22a1 engine (sohc head) and putting a h22 head (dohc) it work but require a lot of modification.
And sometime there is no modification needed... but i dont remember the make.