frankly, if the client contacted me, i'm pretty sure I would tell them the primary photographer is lying about the hard drive failure.
I would never do any more work for him once I knew that is how he was operating, so preserving any relationship would be of no concern.
the main photog didn't like any of your work but paid you (you are not likely to get hired again is my guess) but he fulfilled his obligation
the couple is now in a no win situation also as a decent sized deposit has been paid on the upcoming wedding pictures (plus they got no engagement photos - at least you think they didn't)
if it were me, and they contacted me, i wouldn't lie and I have them come by and show them - but they would have to deal with the main photog (they can be a future client of yours and there is no reason they can't come by your place and you can show them your work)
my guess, and if i was the client, i would personally fire the photog for lying to me and ask for my money back or goto small claims court to get it
as soon as this photog asked you to lie he has opened himself to serious risk of future business problems