When I hear someone say "coulda, shoulda, woulda" in response to someone stating that they should have done something, I think it's a bit too quick a brush-off. If you make a mistake, you should try to discern what preventative action can prevent that mistake in the future. Sometimes the preventative action will be too costly, so will not be undertaken, but to *always* respond with "coulda, shoulda, woulda" to the suggestion is stubborn and unhelpful. Someone who refuses to change any actions to avoid future mistakes has decided never to learn again, or at least never to learn anything practical.
Mokalus of Borg
PS - They're really saying "can't, shan't, won't".
PPS - And that gives a whole new feel to the exchange.