We are contacting you regarding a past Prime Video purchase(s). The below content is no longer playable on Prime Video.
In an effort to compensate you for the inconvenience, we have applied a £5.99 Amazon Gift Card to your account. The Gift Card amount is equal to the amount you paid for the Prime Video purchase(s). To apologize for the inconvenience, we've also added an Amazon Gift Certificate of £5 to your account. Your Gift Card balance will be automatically applied to your next eligible order. You can view your balance and usage history in Your Account here:
What are you talking about? Amazon's digital video purchases don't require any monthly access fee. He paid £5.99 with the idea that he'll get to keep it indefinitely, just like a physical DVD. I don't get why you think it is ok for a seller to revert the sale of a digital item at any time for just the purchase price + £5 but (I presume?) not other sales?
Ignoring all the rest, the issue is simply that it was never bought. He bought a licence to access it. If he expected to "keep" anything, he was an idiot.
I don't think it's okay, but if I'm dumb enough to waste money on this stuff, it's nice to get something extra for my stupidity.