Oh, NightNurse, please don't feel guilty.
I don't know much about ARDS or the stats about prognosis, but you could not have predicted that this would happen to your family member.
It also sounds like he was facing a surgery that he didn't have much choice about undergoing. Of course you told him he'd be fine...that's what people need to hear, when they are scared. It's not wrong to believe it, and you can only focus so much on what *could* go wrong...it is important, IMO, for someone to go in to a surgical situation with as positive an attitude as possible. Even if you HAD feared a poor outcome or complications, could you have pictured saying to him, "things aren't going to go well afterwards?" Of course not. I believe it's one of those "little white lies" done to benefit the person it's told to. I don't think, though, in your case, that you were doing that even.
It was a complication you couldn't have predicted, and I believe that your family member, in the back of his mind, was aware that complications were a possibility with such a major operation. I don't believe for a second that he thinks you weren't being truthful with him, or blames you in any way, shape, or form, for his NOT being "fine" post op, so you shouldn't blame yourself, either.
I hope this makes sense. I wish your family member (and you!) the best.