How about a very polite version of;
I can't tell whether you intend to decline the job. If so, asserting that you're not competent in the technology is a good excuse to decline. If it's true, you probably shouldn't do #3.
In terms of what to say, I'd start with asking questions to try to resolve some of the unknowns. You can explain that t6he company may have publicized the invention in some way, particularly if it didn't think there was any obligation to not publicize it. Any publication of the invention by the company can make the invention unpatentable. You can also explain that failure of the company to publicize the invention in any way (including trying to sell it to the public) indicates that the company (at least that particular company) didn't see significant value in it.
You could also explain that the long time that passed between initial demonstration and filing the application could be problematic -- there's a lot of time for others to work on solving the same problem and accordingly increased risk of other prior art. There may be laches issues to look into.
Your suggestion regarding DIY searching is good. I often explain that searching can be used to reduce uncertainty regarding prior art. I also explain that you can never completely eliminate uncertainty and that reduction of uncertainty follows a curve of diminishing returns. You stop searching when further reductions in uncertainty are not worth the cost of searching further -- another business decision. If Doug Inventor has limited funds, doing the search himself is cheap and he'll understand much more about his prospects by seeing what's out there and what others are doing to solve the same problem. He should probably search specifically for things by the company to which he had disclosed his early prototype. A professional search might be warranted -- another business decision.
One thing I'd try to make very clear is that patents are not cheap and that they're full of risk. If he decides to go forward and you agree to help, you should remind him of all your warnings in writing, e-mail at least.
If you decide not to agree to help, you can consider referring him to someone else. I've had potential clients who were particularly troublesome. As an example, one was in and out of a psychiatric hospital and accepted an offer I never made of $200,000/month for being my office manager -- but she wanted to discuss the benefits package. This was about 4 years ago, and I still get calls asking for a cash advance on her salary or for back salary that I supposedly owe her. I would not refer someone like that to colleagues with whom you'd like to maintain a relationship.
Lastly, I wouldn't recommend #3, unless it's for saving money. You're not going to avoid liability by not actually filing the application. And, if the applicant is to watch their own calendar, they'll likely screw it up and can still blame you. Even if they don't successfully convince any authority that the calendar screw-up is your fault, you really don't want to go through the process of defending yourself.
I hope that helps.
Regards.