I think that it is unwise to try and defend this without proper legal representation. The District Court can be a bit wild West or more akin to rolling a dice
Did the poor advice cause the defendant actual financial loss ? If yes, consider threatening to counterclaim for professional negligence if the quantum of the loss is within the DC jurisdiction.
The counterclaim could be grounded on breach of contract. The breach would be failure to render proper competent professional advice and that would be the professional negligence aspect of matters.
A letter intimating a professional negligence claim and requesting that the matter be referred to the accountant's professional indemnity [PI] insurers might soften their cough. This will receive little or no respect from a legally unrepresented party but if it comes from a solicitor it might be taken more seriously. ( BTW the accountants PI insurers may already know about it as some PI policies provide cover for legal costs for actions against clients for unrecovered fees.)
Legal costs generally follow the event. I don't know what the District Court judges do but elsewhere any decision on costs is at the discretion of the trial judge.
Budget for an appeal to the Circuit Court. With respect to DJs you are more likely to get a better run in the Circuit Court for a matter like this.
I do not know from the details if it is relevant but could the problem with the accountant fall across any of the areas that would form a basis of complaint to their professional body ?
Consider quantum meruit [as much as is deserved]. If some of the performance was proper and adequate the defendant may be expected to pay for that work which was done properly.
Finally, if possible, these things are best settled by negotiation before it all gets out of hand. DC cases are a pain and are sometimes more trouble that a decent High Court case...........