If the solicitor got the rate wrong, that is certainly annoying, but his mistake has not actually cost the couple any money. If the correct rate is 6%, then they are liable for 6%, - end of story. The fact that the solicitor incorrectly quoted 3% does not make him liable for the difference (no more than it would entitle him to keep the surplus if he had inadvertently quoted too much for stamp duty)
However, if the solicitor's delay were to result in a stamp duty penalty (or interest), the solicitor would have to pick up the tab for the penalty. This happens every solicitor at some stage in his\her career.
I rather suspect that the solicitor has sent in the 3%, the Revennue have come back and queried it and the solicitor has in turn written out to the clients. This could easily take close enough to 6 months - particularly if the solicitor queried the matter with Revenue (though I must say it seems pretty straightforward).