Someone I know was recently bitten by a friends dog, so badly that it required surgical treatment. The owners of the dog have not done anything about it as they say it was a 1 off occurrence. I think any responsible owner should have had the dog destroyed as if it's bitten once it could do it again and next time it could be a child. WWYD?
If by done nothing you mean changed absolutely nothing so that similar circumstances are likely to happen again and therefore the dog is likely to react in the same way with the same results then yep, they're completely irresponsible.
It also kind of depends what those circumstances were...
I name changed the other day and this is my first post with this name so entirely coincidental! I think it's hard to comment without knowing the full story, the build up to the incident etc . I've been bitten lots of times, eg. breaking up dog fights, been chased and bitten by a dog when I was on a bike, bitten by a dog on my paper round when I was young. None of these incidents made me think the dogs would have to be destroyed.
I think any pet dog should be given the benefit of doubt in case it was ill or in pain and nobody realised for example. But after any incident like this I would be very carefully managing it's interactions with people. But no, I would not be destroying a dog who had bitten someone once I don't think. If it was a family dog and bit my child, I'd look to rehome it because I would not be able to live comfortably or keep them apart at all times as I would want to after an incident.