Well, like with most social rules of thumb, it's not set in stone. But the basic idea is that hot women don't really have to try hard to attract a mate, so they don't put much effort into the process. The less aesthetically attractive a woman is, the more effort she has to put in. However, it's not an inversely proportionate thing; a balance must be struck. So, given your typical 1-10 scale of hotness with 10 being the hottest, and a choice between three chicks -a 3, a 7 and a 10- go for the 7.

Then i expect he was very disappointed.