I have not been involved in the controversy, but based on what I've heard and read and since you asked "What more can I do?", at your invitation I offer the following suggestions:
1) You and all the other Tartan dealers should contact Tartan/Novis and pressure them to quickly resolve the pending litigation concerning Tartan warranty issues. This resolution should include a full, complete, and unqualified satisfactory outcome for the customers.
2) Do not let Tartan/Novis put a gag order on the final settlement. Full disclosure of the outcome is critical for restoring customer confidence.
3) Insist that Tartan perform the biggest Mea Culpa ever known in the marine industry. Have them accept responsibility, ask forgiveness, and assure current owners and potential customers that this regrettable chapter is an aberration. Pledge to treat all customers like #1 in the future.
4) Urge Tartan to hire as a consultant a qualified marine engineer/naval architect to review quality issues and assure everyone that design, construction, and management problems have been resolved.
5) Make sure Tartan/Novis sacks whoever is responsible for this fiasco. It should never have gotten this far along.
Personally, I find many of the Tartan designs to be very appealing, notably the new 4300. It has been very disheartening to watch this situation unfold. I do not fault you for stepping up and backing the boat you have listed -- if I were the seller I'd greatly appreciate your efforts. But Tartan needs to get past this, or unfortunately the problem will continue to spread to folks like you and your customer. The power to lift the curse rests solely with Tartan/Novis.
Hey ... I asked what more could "I" do for my client and a prospective purchaser. I think that your suggestions are a bit unreasonable for ME to take on. Please consider that I am just one independent dealer, not the company!