He needs to edit his grub config and put in a delay to see the grub
boot menu. Open a terminal and type:

gksudo gedit /etc/default/grub

and look for the line "GRUB_TIMEOUT=0"

Set it to something higher like 5 seconds or so as follows:
GRUB_TIMEOUT=5

Save the file, close it, and in the terminal type 'sudo update-grub'.
On rebooting, he should now see the grub splash screen. Assuming the
Vista partition is still there safe and sound, if it does not show up
as one of the options on the boot grub menu, it may need to be added
manually.

See here for more details on configuring the Grub2 boot menu:
http://ubuntuforums.org/showthread.php?t=1195275