You have 30 days from the first picture to claim the photopass card ID number on your disneyphotopass.com account.
Once you've entered the ID numbers onto your account, it ends that 30 days, and starts a new 30 days. By the end of those 30 days, you have to have ordered something, as your photos will expire.
So if you are still in the 30 days and your friend will be too, that first 30 days still is in effect. If your friend is going outside of 30 days and you haven't yet claimed your card ID number online, I'd do it at about 28 or 29 days to make sure your pictures still exist.
Let's pretend like YOU are going back instead of talking about someone else. If you were going back, you could take your card or get new ones (you could get a new card every day if you wanted to...you don't have to use the same card at all), have your pictures taken, etc. Then put the card ID number(s) on your account, and as long as you get everything done and ordered inside the second 30 day window after putting your card onto your account, all pix should be on the CD and it should all be fine.
You can even spend about $20 more and get a second, official, CD sent to you for the other family.
Hope that wasn't too confusing.