This isn't a question of ethics. This is a question of whether you and the company you're building is worth the talent you have put together to build it. If, particularly at this early stage (the article says the silly words "this late in the game", as though applying to YC is the finish line) your partner were to come to the conclusion that another company were a better opportunity, then your problem is with your business or with your partner. It has nothing to do with other possible businesses that might want your partners talents on their side.

If you're not prepared to deal with the problem of getting and keeping good developers in a highly competitive market, then you're not equipped to deal with running a business in the tech industry. You will be astounded by how hard it is to recruit the very best talent. Middle tier and lesser developers are abundant...it's the cream of the crop, the people who deliver solid, reliable, working code day in and day out, that are difficult to find and to keep.

Welcome to the real world of business, where you have to worry about things like that.

"If you're not prepared to compete in business, whatever the threat, you're toast."

"Poaching my people is crossing the line."

Those two statements look juxtaposed to me. It sounds to me like you're the one who isn't prepared to compete if you aren't willing to pose such a threat to your competitors, or to contend with it when you're on the receiving end.