Sounds too me like the prospect made the decision with whom they felt better and more comfortable with joining and building a business with.
That is one of the draw backs, kinda, of google. You find something, tell someone about it, and they go and google it, then, some other marketer has did a review or something about the company or product, and the prospect potentially signs up with someone else, yet you introduced them to the venture.
Personally I don't see anything unethical about it, IF, the prospect sought you out.
Something I might do is, ask the prospect how they found me, but as you said, they found you through google, and, I'd might proceed to see if they had already talked with someone about the venture, just to keep the air clear.
But ultimately, it really comes down to the prospect's decision whom they join with.