That's sad. :'( Maybe the bunny was ill, not that i'm excusing the pet shop in any way, i'm not. They should health check all their animals but as everyone knows they don't >
We got a bunny from one too & he was very underweight & wasn't eating the food they said he was, he's now putting on weight (thankfully).
I hope your friend went back to the shop & told them what had happened & also complained to their head office or manager, although it probably wouldn't make a difference as so many are only concerned with making money as to them it is purely a business.
I hope your friend is ok & takes some comfort in knowing the little bunny was loved for the short time they had him/her