This was so stupid: did you lie about the dress? Did you force him to buy it at gun point? No. So what's the problem? He bought the dress because he liked it and thought it was worth the price, that's the end of it. It is to some extent expected that the people working there will try to push on you their most expensive stuff, since most of their income is from the commissions.
It's much, MUCH worse that you got your hands on his personal data and infringed on his privacy for that moronic reason. THAT would be reason enough to drop his business with the boutique...