There's a lot to unpack here. One thing we can say at the outset is that settlement of this alleged debt is going to be cheaper than paying a lawyer to deal with it.
The attorney closing on this sale should have gotten a letter from the HOA saying what dues were owed and what they'll be going forward.
If they actually have sued (is it a summons you got or a notice of lien?), then it's true that your client must file an answer or lose by default. He really should consider contacting the plaintiff's attorney in the lawsuit via the information under the signature line and coming to some settlement in return for a full release.
Best of luck.
No attorney-client relationship or privilege is formed by this communication. This answer is to be considered as a general discussion of legal principals and is NOT LEGAL ADVICE. If the asker seeks specific legal advice, he should retain an attorney, who will naturally take the time to consider all aspects of the case and perform any necessary legal research.