Lacky Doc @ "5.6.2 Using XDM"

[perryh at pluto.rain.com]

> You have just about any right in single user mode. Type
> chmod 644 /etc/ttys
> If you edit the file using vi and it isn't writable you can force to
> write to it by typing :w!

Not if his FS is mounted read-only.

Sure, he can remount it read-write, if he knows how :)
(Someone else already posted those instructions;
no need to repeat them here.)