Only one hitch with that.
If he is using a Vista OEM edition, while he can install XP, XP will
overwrite the boot track. And the XP bootloader can't do Vista. So, it
sort of paints one in a corner. Yep, both on the disk but the bootloader
can only do XP.
But, he could Use Ubuntu's Grub bootloader to avoid a repurchase of Vista
full version. Lets assume it could look like:
Partition 1: OEM Recovery partition for Vista, usually about 9GB or so.
Partition 2: Vista OS
Partition 3: XP OS
Partition 4: Unbuntu w. Grub.
The key is to shrink up Partition2 to an appropriate size leaving room
for XP and Ubuntu. The steps might go:
1) backup your data (always a good idea)
2) Shrink partition2 by booting Vista and using diskpart.exe or the GUI
to shrink it leaving enough space for the planned partition 3 and 4.
3) Install XP using only the free space needed and leaving enough for
Ubuntu.
4) Then install Ubuntu in the remaining space.
5) On power up boot, select your OS
You can also with Grub, located in the Ubuntu /boot area alter which OS
is default and the delays.
I call it the triple boot, unlike the triple dip which needs a full
version of Vista, a full version of XP and the OEM Vista isn't needed.