Its not very hard to make your intentions clear outright. Like other siblings have pointed out, it was the way that he propositioned her that was creepy and unprofessional. He appears to be leveraging the financial power he has to get the lady to go out with him, whether he realized that or not. (Strictly personally, my opinion is that he was fully aware of what he did. He seems like a very smart and determined person and its hard to see him not realizing this).

A better way would have been to make it explicit. i.e. "Hey X, now that we are done with business, I was wondering if you would like to meet over coffee sometime later. This will certainly not affect our professional relationship either way, but I think you're an interesting person and would like to know you better."

Some relationships though are just really dicey, like the one above. Its like having a relationship with your direct manager, since the manager is evaluating you professionally, and that's why its generally considered a bad idea.